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  • Writer's pictureNicola Barclay

The relationship between sleep, appetite, and weight gain: here’s what you need to know

Ever wondered how sleep can help you to lose weight? Well, in this article I’ll uncover some of the fundamental links between sleep and appetite-regulation to show just how important getting enough sleep can be in maintaining and managing a healthy weight.

Whilst we might traditionally think the risk factors for weight gain and obesity are eating too much and exercising too little, these factors alone cannot fully explain how we pile on the pounds. In fact, a study combining results of multiple studies, showed a whopping 38% increase in the incidence of obesity in individuals sleeping between 5-7 hours per night. On the other hand, another study showed that sleeping longer than 9 hours was also associated with greater risk of obesity, but to a lesser extent than sleeping too little.

But you may be surprised to know that it’s not only about the amount of sleep that is important when it comes to weight management, but also the timing of sleep.

Our brain and body keep in sync with the 24-hour light-dark cycle by our internal biological clock – also known as our circadian rhythm. This biological clock controls not only when we should be awake and when we should sleep, but also the timing and balance of almost every physiological process in our body – including when we are hungry and when we are full; what food we crave and what foods we choose to avoid; and optimum times for energy expenditure and storage.

Sleeping at times consistent with your internal biological clock is fundamental to keep the physiological processes involved in energy balance and metabolism ticking appropriately.

Impact of insufficient sleep

So how does insufficient sleep contribute to weight gain? Well, during a typical 24-hour day we have a predictable pattern of hormones involved in appetite regulation – ghrelin, the ‘hunger hormone’, which stimulates appetite and hunger; leptin, which is the ‘satiety hormone’ and gives us a sense of feeling full; and PPY which reduces appetite.

A good night’s sleep, at our usual bedtime, keeps levels of ghrelin and leptin nicely regular to reduce hunger during the night, to speed up metabolism and to increase hunger as the morning approaches. We typically see ghrelin levels increase in between meal times, reduce after meals, increase before bedtime, and reduce again in the second half of the night.

Generally speaking, we see lower levels of leptin during the day, and higher levels when we are asleep at night, peaking in the first few hours in the sleep period, and gradually decreasing as the night goes on. On the other hand, levels of PPY are higher during the day, stimulating that feeling of fullness, and lower across the sleep period at night, potentially driving hunger when we wake up.  But when sleep, or the timing of sleep are disrupted, so too is the balance of the appetite-regulating hormones.

To some extent, when we are awake during the night, we do see an increase in energy expenditure, of around about 7% compared to a blissful night of sleep when energy expenditure is reduced. Then why might we see that not getting enough sleep is associated with weight gain rather than weight loss? Well, it is because the amount of energy that we expend during reduced sleep is only equivalent to about 100 calories burnt, but the disruption in the appetite stimulating hormones means that we consume far more than what we expend.

Indeed, if food intake were restricted, we would see the scales tip the other way, but the increase in hunger due to the dysregulation of the hunger and satiety hormones makes us reach into the fridge for food. And the amount of food we consume outweighs the amount burnt during insufficient sleep loss, such that we gain weight rather than lose it.

But it is not just the dysregulation of the appetite-regulating hormones that contributes to weight gain under conditions of insufficient sleep, but psychological mechanisms underlying food choice. During conditions of insufficient sleep our brain circuitry shows increased activation of brain areas associated with reward, and the craving of high-calorie, high-carbohydrate and high fat, sugary foods. Doughnut anyone?!

Additionally, we see the brain circuitry associated with pleasure light up so that we see an increased amount of eating for pleasure (hedonic eating) regardless of need. These associations appear to be particularly higher in women than men, putting them at relatively increased risk of weight gain when not getting sufficient sleep.

Another mechanism linking insufficient sleep and weight gain is the timing of food intake. When we are sleep deprived, we have a tendency to eat later and later in the day, particularly so in the evening hours before bed. One study showed that individuals who eat the majority of their food in the evening are less effective at losing weight than individuals who consume most of their calories earlier.

It is also possible that when we haven’t had enough sleep, we reduce our physical activity due to feelings of tiredness and fatigue. We’re less likely to feel like going to the gym to work out when we’re sleepy, but rather opt for more sedentary behaviour like watching TV, further increasing the likelihood of weight gain.

Impact of sleep at the wrong time

But not only does insufficient sleep contribute to weight gain, so too does sleeping at times inconsistent with our biological clock. We are hardwired to sleep at night, when the sun goes down, and be awake and active during the day. Our biology is optimised to this predictable 24-hour light-dark cycle. However, the modern world, in particular the emergence of electric lighting, has tipped the sleep-wake cycle on its head, enabling us to live and work during the night time, and consequently disrupting our internal biological clocks.

Not surprisingly, in addition to insufficient sleep, the timing of sleep has an impact on appetite regulation, energy expenditure and food intake, and this has important considerations for individuals who regularly tip their days and nights upside down, such as shift-workers.

One study showed that when working during a night shift, energy expenditure across a 24-hour period was reduced compared to the amount of energy expended during a typical day-night cycle. Thus, regular night shift workers may be more likely to gain weight, obesity, and other metabolic disorders as a result of their shifted work lives.

Circadian misalignment during shift work may also predispose to weight gain due to a reduction in physical activity when awake at night, and choosing more calorie rich, unhealthy foods, as we do when sleep-deprived. Furthermore, studies in animals have shown that, even when the same calories are consumed, mice that eat during their biological night are more likely to gain weight than mice that consume the same amount during the day.

Circadian misalignment also alters levels of the appetite-regulating hormones, such that the satiety hormone, leptin, is reduced, promoting the drive to eat, and evidence suggests that the change in the regulation of the appetite hormones under circadian misalignment is greater than when we get insufficient sleep. So the timing of sleep is more important than the absolute amount of sleep we get.

And we’re not just talking about dramatic changes to sleep that shift workers undergo. Small changes in our sleep timing, such as staying up later than our habitual bedtimes, or frequently traveling across time zones increase our social jetlag – the mismatch between our internal clocks and our social worlds. Individuals with high social jetlaghave been shown to be associated with obesity.

But there is a vicious cycle at play. Whilst insufficient sleep is associated with weight gain and obesity, we also know that obesity itself is associated with poor sleep and sleep disorders such as obstructive sleep apnoea.

It should, however, also be noted that sleeping too long is also associated with risk for obesity, so the associations are a little more complex. Individuals with obstructive sleep apnoea typically have excess abdominal fat and large neck circumference, putting them at risk for the collapse of the upper airways which is the hallmark of the disorder. In some individuals, healthy weight loss regimes can be helpful in breaking the cycle and getting sleep back on track.

Whilst there is still much to be learnt about the relationships between sleep and weight gain, the take home messages are clear: prioritise getting sufficient sleep, at times in line with your biological clock, ideally during the night, and eat early!

Much of the content of this review stems from Chaput et al 2023.

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