Why Sleep is Your Metabolic Superpower

We tend to think of sleep as rest – the way we replenish energy.  In truth, your sleeping hours are a highly productive repair shift, especially for your metabolism. 

Each night, your body resets blood sugar, clears metabolic waste, restores energy and even rewires memory. Consistently missing out on quality or quantity of sleep means less of that vital repair work gets done.

Most people notice tiredness after a bad night, but few realise the impact it has on their blood sugar, metabolism and even body composition.

So in our last article we explored melatonin’s role in brain repair, in this part 2 we look at how poor sleep throws off your body’s entire metabolic rhythm – from blood sugar to fat storage.

(When we talk about poor sleep, we mean getting less than seven hours a night, sleeping at irregular times, or waking often through the night – all of which disturb the deep, restorative phases your brain depends on.)

Sleep and insulin: two sides of the same coin

Deep, unbroken sleep keeps your cells sensitive to insulin, the hormone that allows glucose into cells to make energy. Cut the night short and this system falters. Just one poor night can reduce insulin sensitivity by about 25 per cent (1).

That means glucose lingers in the bloodstream (creating inflammation over time) while your brain cells are left hungry for fuel.

The result? Brain fog, irritability, and a body craving quick fixes – sugar, caffeine and refined carbohydrates. You’ll have felt this yourself: after a poor night’s sleep, you wake up wanting pastries or toast, not eggs and greens.

The “tired brain” that acts diabetic

When the brain can’t get enough glucose, it flips into survival mode.

Stress hormones like cortisol and adrenaline surge to keep you going, but they also spike blood sugar and wreck the next night’s sleep (hello, 4 a.m. wake-ups).

Brain scans show that after even a single sleepless night, glucose metabolism in the prefrontal cortex, the region responsible for focus and decision-making, drops sharply (2).

It’s a vicious cycle: sleep loss drives insulin resistance, which drives stress and sugar intake, which drives more sleep loss.

Poor Sleep Changes Your Metabolism

It’s easy to see how poor sleep doesn’t just fog your mind – it rewires your metabolism. Short sleep duration is now recognised as one of the strongest lifestyle predictors of weight gain, insulin resistance and type-2 diabetes – even when calorie intake stays the same,

Even a few nights of shortened sleep raise ghrelin, the hunger hormone, and suppress leptin, which signals fullness (7). The result is stronger cravings for quick-release carbs and sugary snacks, precisely the foods that destabilise blood sugar and accelerate insulin resistance. At the same time, sleep loss changes how your body stores fat: studies show it increases visceral fat, the deep belly fat that drives inflammation (8).

Over time, this mix – more hunger, higher insulin, greater inflammation – pushes many people toward weight gain, pre-diabetes and, eventually, cognitive decline.

So if you’re trying to lose weight or steady your energy, don’t forget about sleep.

High blood sugar, low cognition

Poor sleep raises blood sugar, and when glucose stays high, the brain pays the price.

Overtime poor sleep raises blood sugar, and when glucose stays high, the brain eventually pays the price. Chronically elevated HbA1c, measured in our DRIfT test, predicts faster cognitive decline and higher dementia risk. The same metabolic stress that drives weight gain and diabetes also drives neurodegeneration. That’s why people with insomnia or sleep apnoea are far more likely to develop both type-2 diabetes and Alzheimer’s (3, 4).That is why we cover both sleep and insulin management as a key part of our COGNITION 6-month brain upgrade programme (available to all FRIEND’s of Food for the Brain) – because protecting your brain is possible when you know what to focus on.

The night-shift hormones that matter

Melatonin isn’t just for sleep – it fine-tunes your body’s glucose rhythm and acts as a powereful antioxidant. When evening light suppresses it, next-morning blood sugar shoots higher (5).
Cortisol should fall overnight so insulin can do its work; if stress, late eating or light keeps it high, blood sugar stays stuck.
Growth hormone, released in deep sleep, repairs tissue and builds lean muscle, your natural blood-sugar buffer.

Together these hormones keep the night restorative and the brain calm. Disrupt them and the same chemistry that fuels diabetes starts fuelling Alzheimer’s (6).

Simple Ways to Turn Sleep into a Metabolic Superpower

Guard your 7–8 hours. Deep sleep is where metabolic reset happens.
Skip caffeine or alcohol late. Both fragment sleep and blunt insulin response.
Finish eating at least three hours before bed. Giving your body time to fast allows insulin to fall and encourages fat use for fuel overnight.
Start your day with light, not sugar. Early daylight synchronises your circadian rhythm, boosting morning cortisol naturally so you rely less on coffee and quick carbs.
Pair protein-rich, low-GL meals with consistent sleep. Balanced blood sugar by day supports stable melatonin and growth hormone at night, a feedback loop that keeps your metabolism working for you, not against you. Find 100+ delicious recipes here.https://foodforthebrain.org/uybcookapp/

Sleep as metabolic medicine

Sleep isn’t a luxury or a waste of time –  it’s your brain’s way of resetting and restoring the entire body. It shapes body composition, curbs cravings, steadies energy and supports the metabolism that powers your mind.

Takeaway: good sleep, like good nutrition, is prevention in action.
Want to dive deeper? Join us for the Sleep Solution Webinar with sleep scientist Greg Potter. Find out more here

Food for the Brain is a not-for-profit educational and research charity that offers a free Cognitive Function Test and assesses your Dementia Risk Index to be able to advise you on how to dementia-proof your diet and lifestyle.

By completing the Cognitive Function Test you are joining our grassroots research initiative to find out what really works for preventing cognitive decline. We share our ongoing research results with you to help you make brain-friendly choices.

Please support our research by becoming a Friend of Food for the Brain.

Reference:

Spiegel K et al. Impact of sleep debt on metabolic and endocrine function. Lancet. 1999;354(9188):1435–9.

Benedict C et al. Acute sleep deprivation reduces energy expenditure and brain glucose metabolism. Sleep. 2012;35(7):981–8.

Yaffe K et al. Sleep duration and risk of type 2 diabetes: a meta-analysis. Diabetes Care. 2015;38(9):1633–40.

Sabia S et al. Association of sleep duration in middle and old age with dementia incidence. Nat Commun. 2021;12:2289.

Gooley JJ et al. Exposure to room light before bedtime suppresses melatonin onset and shortens its duration. J Clin Endocrinol Metab. 2011;96(3):E463–72.

Musiek ES, Holtzman DM. Mechanisms linking circadian clocks, sleep, and neurodegeneration. Science. 2016;354(6315):1004–8.

Spiegel K et al. Brief sleep curtailment decreases leptin, increases ghrelin, and causes increased hunger and appetite. Ann Intern Med. 2004;141(11):846–50.

Nedeltcheva AV et al. Insufficient sleep undermines dietary efforts to reduce adiposity. Ann Intern Med. 2010;153(7):435–41.

By Patrick Holford

What does any animal, perhaps your dog, do after exercising or going for a walk?

Sleep.

Sleep is how the brain recovers. There is now overwhelming evidence that sleep is a ‘brain essential’ and just like Goldilocks, it seems we need just the right amount. Getting too much, or too little, increases our risk for cognitive decline.

The optimal amount of sleep for brain health appears to be a total of seven hours. This does not necessarily need to be in one uninterrupted stretch – a study found that napping after physical activity can reduce the risk of cognitive impairment (1).

However, those consistently getting less than seven hours of sleep may be doubling their risk of age-related cognitive decline (2). A UK study of Whitehall civil servants, which began in the 1980s, found that persistent short sleep at ages 50, 60, and 70 was associated with a 30% increased risk of dementia (3). Sleep loss does not just increase long-term dementia risk – it also reduces empathy, increases negative emotions, and impairs next-day functioning (4).

Why Sleep Is Essential to Brain Health?

Think of sleep as the brain’s housekeeper. During sleep, circulation of blood and cerebrospinal fluid improves, helping to clear out waste products from brain metabolism (5). These include harmful oxidants and amyloid protein, the latter linked to Alzheimer’s and brain inflammation – which can begin accumulating after just one night of poor sleep (6).

One key agent in this nightly brain cleanse is melatonin. As night falls, our brains convert serotonin into melatonin, primarily in the pineal gland – referred to by Descartes as the seat of the soul, and known in yoga as the ‘third eye’ chakra.

Sensitive to light via receptors behind the eyes, the pineal gland is the only endocrine organ in direct contact with the external world. Darkness triggers melatonin production, while exposure to light – including screen use before bed – suppresses it.

Melatonin helps keep us in sync with the circadian cycle. Some frequent flyers even use melatonin supplements to overcome jet lag and adjust their sleep rhythms more easily (7).

More than just a sleep aid, melatonin acts as a powerful antioxidant – disarming damaging oxidants, restoring mitochondrial energy production, and acting as an anti-inflammatory. It has been used to support recovery in cancer, COVID-19, and cardiovascular conditions (8,9). Reduced brain melatonin levels and circadian disruption are also observed in individuals with cognitive decline.

Why Dreaming Matters?

Sleep isn’t just for rest – it’s a deeply active process. About 30 minutes after falling asleep, we enter deep sleep, marked by slower breathing, a reduced heart rate, and lower blood pressure. This phase restores and repairs bodily tissues. About 90 minutes in, we shift into REM (rapid eye movement) sleep – where most dreaming occurs.

REM sleep is critical for brain health. Each night, we cycle between deep, light, and REM sleep three to five times, with REM ideally making up about 25% of total sleep.

REM and deep sleep phases also see increased production of growth hormone, which supports tissue repair. Meanwhile, melatonin helps clear metabolic waste. However, under stress, cortisol levels rise and suppress REM sleep and growth hormone production, reducing the brain’s ability to recover. Sleep-deprived individuals tend to experience more REM when they finally do sleep, suggesting REM plays a key role in emotional processing.

One theory suggests that dreams help us metabolise suppressed emotions – fear, anger, sadness – stored during our busy days. If you have a vivid, emotional dream, it may be worth tracing it back to unresolved feelings from the previous day.

How Chronic Stress Disrupts Sleep and Brain Function?

Chronic or intense stress – such as bereavement, illness, or financial strain – has been shown to increase the risk of cognitive decline and dementia (10). However, good sleep can help process a stressful day.

The perception of control matters, too. Studies show that high job demands combined with low control are strongly linked to an increased risk of depression and cognitive impairment (11). Examples might include caregiving for a loved one with dementia while navigating health services, or working in a high-stress job without the resources to make meaningful changes.

Your Brain on Cortisol: The Hippocampus Feedback Loop

Two hormones mediate stress: adrenaline (short-acting) and cortisol (longer-acting). Adrenaline prepares you to act quickly – it’s the fight-or-flight hormone. Cortisol helps regulate energy and alertness throughout the day.

In the morning, cortisol naturally rises to get us going. It should fall in the evening to support sleep. But chronic stress disrupts this rhythm. If cortisol stays high at night, sleep is disturbed. If it’s too low in the morning, you may feel foggy and reach for caffeine.

Excess cortisol impairs memory, slows thinking, lowers social functioning, and raises the risk of dementia (12). What’s happening in the brain is that cortisol overstimulates the hippocampus, which is responsible for memory and emotional regulation. With prolonged stress, this feedback loop fails – the hippocampus shrinks, and cortisol levels remain elevated, accelerating brain ageing.

Short-Term Relief, Long-Term Harm: Sugar and Alcohol as Stress Crutches

Oscar Ichazo described how we reach for compensations under stress. Unfortunately, many – like alcohol and sugar – backfire.

Alcohol temporarily boosts GABA, calming the nervous system and reducing adrenaline. But the effect is short-lived. Drinking too much reduces GABA receptor sensitivity the next day, leaving us more anxious. In the long term, alcohol is neurotoxic and increases dementia risk (12). It also disrupts sleep architecture, impairing the brain’s ability to repair itself.

Sugar triggers dopamine and activates the brain’s reward circuits, making us crave more. It also spikes the adrenal system, amplifying stress and cortisol levels (13). Fats and proteins do not have this effect – this is unique to sugar.

So, when we use sugar or alcohol to manage stress, we often wake up feeling more anxious and foggy. This leads us to reach for caffeine and more sugar, which spikes cortisol again, leaving us even more depleted by evening – creating a cycle of stress, poor sleep, and accelerated brain ageing.

Simple Ways to Break the Cycle

The good news? You can reverse this pattern. Start here:

Become a FRIEND and get access to your personalised COGNITION® programme which which includes:
– A whole module dedicated to sleep and calm
– Another focused on helping you reduce sugar
– Plus monthly live group coaching to help you stay focused and on track
Prioritise seven hours of quality sleep each night.
Identify and reduce common stress triggers.
Be mindful of alcohol and sugar intake.
Find positive outlets: yoga, walking, journaling, a good book – like Upgrade Your Brain.

–

Thank you for reading!
Food for the Brain is a non-for-profit educational and research charity that offers a free Cognitive Function Test and assesses your Dementia Risk Index to be able to advise you on how to dementia-proof your diet and lifestyle.

By completing the Cognitive Function Test you are joining our grassroots research initiative to find out what really works for preventing cognitive decline. We share our ongoing research results with you to help you make brain-friendly choices.

Please support our research by becoming a Friend of Food for the Brain.

———

References:

¹ Qian YX, Ma QH, Sun HP, Xu Y, Pan CW. Combined effect of three common lifestyle factors on cognitive impairment among older Chinese adults: a community-based, cross-sectional survey. Psychogeriatrics. 2020 Nov;20(6):844-849. doi: 10.1111/psyg.12604. Epub 2020 Aug 31. PMID: 32869429.

² Bubu OM, Brannick M, Mortimer J, Umasabor-Bubu O, Sebastião YV, Wen Y, Schwartz S, Borenstein AR, Wu Y, Morgan D, Anderson WM. Sleep, Cognitive impairment, and Alzheimer’s disease: A Systematic Review and Meta-Analysis. Sleep. 2017 Jan 1;40(1). doi: 10.1093/sleep/zsw032. PMID: 28364458.

³ Sabia S, Fayosse A, Dumurgier J, van Hees VT, Paquet C, Sommerlad A, Kivimäki M, Dugravot A, Singh-Manoux A. Association of sleep duration in middle and old age with incidence of dementia. Nat Commun. 2021 Apr 20;12(1):2289. doi: 10.1038/s41467-021-22354-2. PMID: 33879784; PMCID: PMC8058039.

⁴ Krause AJ, Simon EB, Mander BA, Greer SM, Saletin JM, Goldstein-Piekarski AN, Walker MP. The sleep-deprived human brain. Nat Rev Neurosci. 2017 Jul;18(7):404-418. doi: 10.1038/nrn.2017.55. Epub 2017 May 18. PMID: 28515433; PMCID: PMC6143346.

5⁶ Xie L, Kang H, Xu Q, Chen MJ, Liao Y, Thiyagarajan M, O’Donnell J, Christensen DJ, Nicholson C, Iliff JJ, Takano T, Deane R, Nedergaard M. Sleep drives metabolite clearance from the adult brain. Science. 2013 Oct 18;342(6156):373-7. doi: 10.1126/science.1241224. PMID: 24136970; PMCID: PMC3880190.

⁶ Shokri-Kojori E, Wang GJ, Wiers CE, Demiral SB, Guo M, Kim SW, Lindgren E, Ramirez V, Zehra A, Freeman C, Miller G, Manza P, Srivastava T, De Santi S, Tomasi D, Benveniste H, Volkow ND. β-Amyloid accumulation in the human brain after one night of sleep deprivation. Proc Natl Acad Sci U S A. 2018 Apr 24;115(17):4483-4488. doi: 10.1073/pnas.1721694115. Epub 2018 Apr 9. PMID: 29632177; PMCID: PMC5924922.

⁷ Herxheimer A, Petrie KJ. Melatonin for the prevention and treatment of jet lag. Cochrane Database Syst Rev. 2002;(2):CD001520. doi: 10.1002/14651858.CD001520. PMID: 12076414.

⁸ Keithahn C, Lerchl A. 5-hydroxytryptophan is a more potent in vitro hydroxyl radical scavenger than melatonin or vitamin C. J Pineal Res. 2005 Jan;38(1):62-6. doi: 10.1111/j.1600-079X.2004.00177.x. PMID: 15617538.

⁹ Chitimus DM, Popescu MR, Voiculescu SE, Panaitescu AM, Pavel B, Zagrean L, Zagrean AM. Melatonin’s Impact on Antioxidative and Anti-Inflammatory Reprogramming in Homeostasis and Disease. Biomolecules. 2020 Aug 20;10(9):1211. doi: 10.3390/biom10091211. PMID: 32825327; PMCID: PMC7563541; regarding covid see also Tan DX, Reiter RJ. Mechanisms and clinical evidence to support melatonin’s use in severe COVID-19 patients to lower mortality. Life Sci. 2022 Apr 1;294:120368. doi: 10.1016/j.lfs.2022.120368. Epub 2022 Jan 30. PMID: 35108568; PMCID: PMC8800937.; see also Begum R, Mamun-Or-Rashid ANM, Lucy TT, Pramanik MK, Sil BK, Mukerjee N, Tagde P, Yagi M, Yonei Y. Potential Therapeutic Approach of Melatonin against Omicron and Some Other Variants of SARS-CoV-2. Molecules. 2022 Oct 16;27(20):6934. doi: 10.3390/molecules27206934. PMID: 36296527; PMCID: PMC9609612.; regarding cancer see Reiter RJ, Rosales-Corral SA, Tan DX, Acuna-Castroviejo D, Qin L, Yang SF, Xu K. Melatonin, a Full Service Anti-Cancer Agent: Inhibition of Initiation, Progression and Metastasis. Int J Mol Sci. 2017 Apr 17;18(4):843. doi: 10.3390/ijms18040843. PMID: 28420185; PMCID: PMC5412427.

¹0 Franks KH, Bransby L, Saling MM, Pase MP. Association of Stress with Risk of Dementia and Mild Cognitive Impairment: A Systematic Review and Meta-Analysis. J Alzheimers Dis. 2021;82(4):1573-1590. doi: 10.3233/JAD-210094. PMID: 34366334.

¹1 Wang HX, Wahlberg M, Karp A, Winblad B, Fratiglioni L. Psychosocial stress at work is associated with increased dementia risk in late life. Alzheimers Dement. 2012;8(2):114-20. doi: 10.1016/j.jalz.2011.03.001. PMID: 22404853; see also Gonzalez-Mulé, E., & Cockburn, B. S. (2021). This job is (literally) killing me: A moderated-mediated model linking work characteristics to mortality. Journal of Applied Psychology, 106(1), 140–151. https://doi.org/10.1037/apl0000501; see also Gonzalez-Mulé E, Kim MM, Ryu JW. A meta-analytic test of multiplicative and additive models of job demands, resources, and stress. J Appl Psychol. 2021 Sep;106(9):1391-1411. doi: 10.1037/apl0000840. Epub 2020 Sep 21. PMID: 32955269.

¹2 Ouanes S, Popp J. High Cortisol and the Risk of Dementia and Alzheimer’s Disease: A Review of the Literature. Front Aging Neurosci. 2019 Mar 1;11:43. doi: 10.3389/fnagi.2019.00043. PMID: 30881301; PMCID: PMC6405479.¹3 Gonzalez-Bono E, Rohleder N, Hellhammer DH, Salvador A, Kirschbaum C. Glucose but not protein or fat load amplifies the cortisol response to psychosocial stress. Horm Behav. 2002 May;41(3):328-33. doi: 10.1006/hbeh.2002.1766. PMID: 11971667.

Just one night of little sleep can have a significant impact on our mental wellbeing and cognitive function.  Although it may appear that our brain completely switches off whilst we’re sleeping, it is actually performing highly sophisticated tasks and is very far from being inactive. During sleep the brain replays memories from the day, sifting out what is no longer necessary, then consolidating what needs to be kept. It also regulates emotional memory in the amygdala (the emotion centre of the brain). Studies investigating sleep deprived people and those who sleep optimal amounts, demonstrate stark differences in MRI brain scanning between the two groups. Interestingly, what is most apparent is that a lack of sleep can cause a disconnect between the amygdala and the prefrontal cortex, which is the area of the brain that is associated with rational, higher-level cognitive processes that involve controlling short-sighted, reflexive behaviours, favouring problem-solving and self control. So it’s no surprise that with less than optimal sleep, we suddenly become a lot less tolerant to things that may not usually bother us and our cognitive abilities such as concentration and memory also suffer. 

Sleep disturbances are more than often concurrent with mental health conditions and can commonly precede symptoms of mood imbalances, such as depression and anxiety. In the UK, as many as two thirds (67%) of UK adults suffer from disrupted sleep and nearly a quarter (23%) manage no more than five hours a night, which could be one of the many contributing factors of increasing levels of poor mental health. Approximately 1 in 4 in the UK will experience a mental health problem every year and depression has been labelled as the second leading cause of disability globally. Considering the research showing the importance of sleep for mental health and the parallel rise of insomnia and conditions such as depression and anxiety, targeting optimal sleep with simple strategies may prove to have a significant positive impact on mental wellbeing. 

Improving sleep is in fact the biggest health ambition for a quarter (26%) of UK adults, but half (51%) admit that they don’t take any measures to help them sleep. The following 4 steps are simple and practical dietary and lifestyle strategies to help you optimise your sleep and get you started:

1. Complex Carbohydrates for Dinner

With the ever-rising trend of keto diets it’s no wonder we’re terrified of eating foods that are high in carbohydrates. However, there isn’t a one-size-fits-all when it comes to nutrition. For some who may be undergoing a significant amount of stress and insomnia, having the right type of carbohydrate can actually be beneficial and even therapeutic. Have you ever wondered why you crave carbohydrates or sugar when you’re stressed? This is because stress reduces the amount of available serotonin in the brain and carbohydrates help stimulate serotonin production, helping bring on feelings of calm and helping the body and mind relax. Serotonin is also a precursor to melatonin, the body’s hormone and neurotransmitter that induces sleep. So without enough serotonin, we simply cannot make melatonin, which will ultimately lead to problems getting to sleep. However, it’s important to note that we can’t just eat any type of carbohydrate – choosing complex, fibre-rich sources of carbohydrate foods, which provide a sustainable source of energy for the body and prevent blood sugar crashes, can help prepare the body for a better night’s sleep. 

Foods such as sweet potatoes, parsnips, beets, pumpkin, butternut squash, as well as wholegrains like brown rice and oats and pulses like chickpeas and butter beans, are all fantastic sources of slow-releasing carbohydrates. Think about including these in your evening meal along with a protein-rich food to give the brain a little serotonin boost, which will help to relax the body and optimise melatonin levels for a more restful night’s sleep. 

2. Avoid the Night-Cap 

It’s called a night-cap, but it really does little else than fool the body into slumber that is in fact very short-lived. In sleep scientist Matthew Walker’s book ‘Why we sleep’, he says how drinking is more like anesthesia than real sleep, essentially sedating the body. Whilst alcohol may get you off to sleep quicker, it actually prevents the body from entering REM sleep, a phase of deep sleep where we typically dream. This is because when the body is metabolising alcohol, the chemical by-product of this process called aldehyde is created, which is essentially what blocks REM sleep. REM sleep is important for helping to solidify memories in the brain, as well as helping the brain to make connections and identify patterns, thus helping us learn.  

So try to avoid the temptation of a night-cap or the glass of wine to help ease off the stress of the day. Instead, think about what you could have as a replacement. Perhaps a favourite warm drink or even a hot bath with some essential oils to help you relax. 

3. Try a Guided Meditation 

Even just 10-15 mins of bringing awareness to the body and the breath can help to switch on the parasympathetic nervous system, which is what helps to inhibit the stress response. When we are stressed and anxious, our body responds by creating hormones and stimulating neurotransmitters that help mobilise the body for managing life-threatening situations. Overtime, this can weaken our ability to switch off and recalibrate, which can have a negative impact on sleep. In a study where 32 patients with severe chronic insomnia engaged in meditation every evening over the course of 8 weeks, their Insomnia Severity Index (ISI) scores greatly improved (from 20.9 to 10.4). In addition, 21 out of 32 had either stopped the usage of sedative or hypnotic agents to induce sleep or greatly reduced the intake of them. This is just one study of many that have demonstrated positive effects of using different meditation tools to help support sleep. 

There are many apps and online videos to help get you started. For example, Calm and Headspace are just two fantastic apps with guided meditations specifically suited to help encourage restful sleep. Try setting aside just 10 mins before bed to help get into a regular routine. 

4. Avoid Spicy and Acidic Foods at Night

Spicy and acidic foods can, for many, lead to acid-reflux or heartburn. This may be because they have compounds like capsaicin that relax the sphincter (which separates the stomach and the oesophagus), leading to stomach acid trickling into the throat when lying down. If you find you’re susceptible to heartburn, you may want to consider avoiding these foods at night. You might also want to rule out other causes of acid reflux such as food intolerances, which can be tested via various private companies such as York Test and Biolab, if you’re based in the UK.