There's a good chance you don't realise just how important sleep is to every aspect of your health, and to your body composition. Here's what can happen when you don't get enough, how much you actually need, and what you can do about it.
Why sleep matters
Sleep is a chance for your body to reset after the day. Enough restful sleep is essential for all of your bodily systems to recover and work at their best, and that means everything: hormonal systems, appetite regulation, blood glucose control, and muscle growth and repair, to name just a few.
Not getting enough sleep can disrupt all of these systems, and as we'll cover below, that makes it significantly harder to achieve your body composition goals.
Sleep and body fat
One of the most consistently reported effects of insufficient sleep in research is an increase in appetite. Not sleeping enough makes you hungry.
Researchers believe changes in appetite are caused by changes in satiety hormones, specifically leptin, which reduces appetite, and ghrelin, which increases food intake. One study found that a few days of sleep deprivation caused leptin levels to drop by 18% and ghrelin to increase by 28%, resulting in a 23% increase in hunger.1
Other studies have shown that even a single night of reduced sleep leads to increased food cravings, greater food reward in the brain, and a tendency to choose larger portion sizes.2 What's more, the effect is specific: sleep deprivation doesn't just increase hunger in general, it makes people hungry for high-calorie, ultra-processed foods that are high in both fat and carbohydrates.
If you're trying to lose body fat, this is a significant problem. Poor sleep increases appetite and drives cravings for processed food, while we live in an environment where that food is readily available. The result is that not getting enough sleep makes dieting considerably harder than it needs to be.
To make matters worse, studies have shown that if you're losing weight and not getting enough sleep, you're more likely to lose muscle rather than fat. One study found that people sleeping only five and a half hours during a calorie deficit lost 55% less fat and 60% more lean mass than people sleeping eight and a half hours.3
Sleep and muscle building
Poor sleep also makes it harder to build muscle. Even modest, short-term sleep loss, going from eight hours to around six, can lead to increases in pro-inflammatory cytokines in the body, and higher levels of these substances are associated with muscle loss in other studies.4
Sleep loss also disrupts how the body releases hormones. Just two nights of four hours of sleep can lead to elevated levels of cortisol, the stress hormone.5 Chronically high cortisol can increase muscle protein breakdown,6 and on top of that, one study found that eight nights of just five hours of sleep reduced testosterone levels by around 10-15%.7
The effects go beyond the physiological, too. People who don't get enough sleep are significantly more likely to engage in behaviours that work against their goals, such as physical inactivity and poor dietary choices. Poor sleep can also lead to higher levels of stress, which makes people less likely to maintain healthy habits like eating well and exercising regularly.
In short, poor sleep doesn't just make it physically harder to build muscle and lose fat, it also makes you less likely to do the things you need to do in the first place.
For more expert advice on sleep and exercise performance, check out this article:
How much sleep do you need?
The honest answer is that we don't know exactly. But if you regularly struggle to wake up, don't feel rested, and feel tired throughout the day, there's a good chance you're not getting enough.
The common guideline is seven to nine hours per night. If you're physically active or exercise regularly, aiming for the upper end of that range is probably a good idea, since your body simply needs more time for rest and recovery.
One paper compared natural sleep duration in primates, our closest animal relatives, and concluded that, based on various biological factors, humans may naturally require around nine and a half hours of sleep per night.8 While this isn't proven, it does suggest that our modern lifestyle may not be allowing us as much sleep as our biology would ideally call for.
How to improve sleep quality
There are a lot of tips available on sleep, and not all of them will be practical for everyone. A small improvement is still an improvement, so start with what you can and build from there.
Maintain a consistent sleep and wake time
Our bodies maintain a stable circadian rhythm, a cycle controlled by biological clock genes in our cells. Going to bed and waking up at different times disrupts this rhythm and makes falling asleep more difficult. Keeping a consistent schedule is one of the best things you can do.
Avoid large meals close to bedtime
If your body is working hard to digest a heavy meal, you'll find it harder to sleep. Try to eat your last full meal at least three hours before bed and keep it relatively light. Taking protein before bed is probably fine, as it's relatively easy to digest.
Limit caffeine later in the day
Caffeine has a half-life of around six hours, meaning that if you take a pre-workout containing 200mg of caffeine, there will still be around 100mg in your system six hours later. Keep caffeinated drinks, coffee, tea and pre-workout, to the earlier part of the day.
Reduce blue light exposure in the evening
Blue light from the sun plays an important role in maintaining our circadian rhythm, but blue light from electric lights, TVs, computer screens and phone screens late at night can prevent your brain from registering that it's time to sleep. Blue light blocking glasses or apps for your screens can help in the evenings.
Optimise your bedroom environment
Light from street lamps or digital clocks can make sleeping more difficult. Blackout blinds or a sleep mask can help. If your room is noisy, earplugs are worth trying. Temperature also matters: a bedroom that is too hot or too cold can disrupt sleep. Cold feet in particular can keep people awake, and wearing socks in bed may help.
Exercise regularly
Regular exercise is associated with better sleep quality. One caveat: for some people, exercising close to bedtime can make it harder to fall asleep. If that applies to you, shifting your workouts to earlier in the day may help.
Manage a busy mind
If you struggle to sleep because your mind won't switch off, two things may help. The first is writing a to-do list for the following day, so you're not trying to keep track of things mentally overnight. The second is pre-sleep meditation. Despite its reputation, there is a good body of research supporting meditation's benefits for sleep quality, and a number of apps are available to help you build the habit.
Sleep supplements
Many supplements are marketed as sleep aids, but there isn't a great deal of evidence to support most of those claims.
One supplement that has more backing is magnesium. It plays a role in neural transmission in the brain and may genuinely benefit people who are deficient. However, if you're not deficient, supplementing is unlikely to help. If you're concerned about your magnesium levels, look for highly bioavailable forms such as magnesium glycinate or magnesium citrate.
It's also worth remembering that there are likely no quick fixes for poor sleep. Long-term habit change, similar to the sustained changes needed for diet and exercise, is the most effective approach.
To find out what foods are high in magnesium, check out this article:
Frequently asked questions
How does poor sleep affect body fat?
Poor sleep disrupts the hormones that regulate appetite, increasing hunger and driving cravings for high-calorie, processed foods. Research also suggests that insufficient sleep during a calorie deficit leads to a greater proportion of weight loss coming from muscle rather than fat.
How does sleep affect muscle building?
Sleep deprivation can raise cortisol levels, increase pro-inflammatory markers associated with muscle loss, and reduce testosterone levels. Combined, these effects make it significantly harder to build and maintain muscle mass.
How much sleep should I get?
The general guideline is seven to nine hours per night. If you train regularly, aiming for the upper end of that range is advisable, as your body needs additional time to recover.
Does magnesium help with sleep?
It may, but only if you're deficient. Magnesium plays a role in neural transmission and may improve sleep in people with low levels, but there is little evidence it helps those who are not deficient. If supplementing, look for bioavailable forms such as magnesium glycinate or magnesium citrate.
Can exercise improve sleep?
Yes. Regular exercise is associated with better sleep quality. However, some people find that exercising too close to bedtime makes it harder to fall asleep, in which case training earlier in the day is worth trying.
Take home message
Sleep is one of the most underrated factors in body composition and overall health. Not getting enough disrupts appetite hormones, increases cravings for processed food, impairs muscle building and recovery, and makes it harder to maintain the habits that support a healthy lifestyle.
Seven to nine hours per night is the general recommendation, with more likely beneficial for those who train regularly. Focus on consistent sleep and wake times, limit caffeine and screen exposure later in the day, and optimise your environment. Small, consistent improvements will make a bigger difference over time than any supplement.
Enjoy this article? Find more on sleep and wellness support here:
- Spiegel, K., Tasali, E., Penev, P., & Van Cauter, E. (2004). Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite. Annals of Internal Medicine, 141(11), , 846-850.. https://doi.org/10.7326/0003-4819-141-11-200412070-00008
- Yang, C. L., Schnepp, J., & Tucker, R. M. (2019). Increased hunger, food cravings, food reward, and portion size selection after sleep curtailment in women without obesity. Nutrients, 11(3), , 663.. https://doi.org/10.3390/nu11030663
- Nedeltcheva, A. V., Kilkus, J. M., Imperial, J., Schoeller, D. A., & Penev, P. D. (2010). Insufficient sleep undermines dietary efforts to reduce adiposity. Annals of Internal Medicine, 153(7), , 435-441.. https://doi.org/10.7326/0003-4819-153-7-201010050-00006
- Vgontzas, A. N., Zoumakis, E., Bixler, E. O., Lin, H. M., Follett, H., Kales, A., & Chrousos, G. P. (2004). Adverse effects of modest sleep restriction on sleepiness, performance, and inflammatory cytokines. Journal of Clinical Endocrinology & Metabolism, 89(5), , 2119-2126.. https://doi.org/10.1210/jc.2003-031562
- Leproult, R., Copinschi, G., Buxton, O., & Van Cauter, E. (1997). Sleep loss results in an elevation of cortisol levels the next evening. Sleep, 20(10), , 865-870.. https://doi.org/10.1093/sleep/20.10.865
- Katsuhara, S., Yokomoto-Umakoshi, M., Umakoshi, H., Matsuda, Y., Iwahashi, N., Kaneko, H., Ogata, M., Fukumoto, T., Terada, E., Sakamoto, R., & Ogawa, Y. (2022). Impact of cortisol on reduction in muscle strength and mass: A Mendelian randomization study. Journal of Clinical Endocrinology & Metabolism, 107(4), , e1477-e1487.. https://doi.org/10.1210/clinem/dgab862
- Leproult, R., & Van Cauter, E. (2011). Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA, 305(21), , 2173-2174.. https://doi.org/10.1001/jama.2011.710
- Nunn, C. L., & Samson, D. R. (2018). Sleep in a comparative context: Investigating how human sleep differs from sleep in other primates. American Journal of Physical Anthropology, 166(3), , 601-612.. https://doi.org/10.1002/ajpa.23427
