Weight Loss Step 6: How your internal body clock affects weight loss and fat burning

There’s a clock, ticking deep within your brain, that functions with near twenty-four hour precision. It responds to the rising and the setting of the sun so that your patterns of sleep and wakefulness match the rotation of the Earth about its axis. If you want to burn fat as fast as it is feasible and healthy to do so, you had better make sure you are aligned with this powerful timekeeper.

Known as the circadian rhythm (circadian in Latin means ‘about a day’) this biological clock is mysterious, and more than a little otherworldly. It was Charles Darwin who, after visiting the Galapagos Islands in 1835, first realised that life is cyclical, and he later published a treatise on the daily pattern of leaf movements in plants. It is well known now that this invisible clock ticks in most animals too.

You can't beat the clock

Any disruption to this biological timekeeper can have serious consequences for metabolism and hormone secretion. Much of this disruption has been blamed on electric lighting and the 24-hour lifestyle to which we have grown accustomed. We create our own, dissonant settings, which ultimately clash with our clocks and create disharmony in our hormones. The phenomenon of living against the clock has been termed ‘social jetlag’ by scientists, and this form of jetlag has been found to significantly increase the likelihood of being overweight; conversely, adequate sleep has been found to reduce that likelihood. When and how long you sleep, and the frequency and timing of your meals, can all influence the way your body burns or stores your food.

Work and school timetables, and the use of alarm clocks to align wakefulness with those timetables, contribute to the amount of social jetlag accrued by each individual. Social jetlag is at its most extreme during adolescence (they really do need to stay in bed half the morning) but continues throughout the working life.

Here, we report the results from a large-scale epidemiological study, showing that, beyond sleep duration, social jetlag is associated with increased BMI. Our results demonstrate that living “against the clock” may be a factor contributing to the epidemic of obesity.

— Roenneberg et al (2012)

While you were sleeping...

Your metabolic rate falls by around 15 per cent during sleep. If you think about it, this is a surprisingly small reduction for such apparent inertia. But don’t be deceived; as you slumber, blissfully unaware of the workings of your autonomic nervous system, your body is busy carrying out innumerable metabolic processes, and your brain is burning fuel furiously. The human brain consumes in the region of 20 per cent of what you eat, and approximately 16 times more than muscle. No wonder thinking hard can be so exhausting. It's like an intense cerebral workout. Your brain is a fuel-hungry furnace, demanding around 400 calories a day.

Asleep, but hard at work

Look after your brain, and it will repay you in kind. Sleep for seven to eight hours, in a darkened room, and your brain will switch on the hormones that initiate fat burning. At night, when you are in fasting mode, growth hormone (GH) swings into action. This noble, fat-mobilising hormone stimulates the production of chemicals called ketones and the release of fatty acids from your fat stores to provide your brain and the rest of your body with the fuel needed to perform their metabolic functions. The secretion of GH starts at the beginning of the first deep sleep cycle. However, peak GH secretion is inhibited if the onset of sleep is delayed.

Sleep influences the regulation of another hormone involved in fat metabolism: leptin. Leptin (from the Greek word leptos meaning thin) is an appetite suppressant; it is secreted by fat cells and levels peak when you are asleep, which explains why you might go to bed on an empty stomach but do not (unless you have a metabolic disturbance) wake up hungry in the middle of the night.

It is in your interest to keep in step with your circadian rhythms and allow growth hormone and leptin to get on with their jobs. However, if you don’t get enough sleep, they can't. There is an interesting piece of research that has been ongoing since 1993, called the Wisconsin Sleep Cohort Study. One notable finding to emerge from this continuous research is that a loss of 3 hours sleep (five hours instead of eight) is associated with low leptin and an average four to five per cent greater body weight.

Keep in step

The whole world is getting fatter - prevalence of obesity across the globe has doubled since 1980 – and at the same time, there has been a parallel trend in reduced sleep duration. There is growing and convincing evidence that the two phenomena are linked. Consider the US, where obesity is an even bigger crisis than it is here in the UK (but we’re catching up fast). Americans are sleeping less. A 1960 survey conducted by the American Cancer Society found that on average people most commonly enjoyed between eight and nine hours sleep a night. Fast-forward to 1995, when another survey, this time by the National Sleep Foundation, found that sleeping for seven hours was the norm. More recently, research has found that six or fewer hours is common.

The majority of studies (and there have been around 50 worldwide) looking at the link between sleep and obesity in adults and children have found a significant association between short sleep duration (less than six hours) and increased risk of obesity. Duration of six or fewer hours has also been associated with other health problems, such as diabetes. Every night, as you sleep, your body balances its energy accounts and in the morning you wake up and discover how the books are looking.

A word to the wise...

Don’t be a night owl

Unfortunately, it's not just lack of sleep that can upset the circadian rhythm and lead to weight gain; so too can sleeping at the wrong time. When mice, which are nocturnal mammals, are fed during the day (when they would normally be asleep) they gain significantly more weight than mice fed only during the night, when they are normally awake and feeding. The same has been found in rats, which are prone to obesity when given food at a time when they would normally be asleep. Human shift workers are no different: studies consistently show that overweight and obesity are far more prevalent in shift workers than day workers, and that shift work is a risk factor for becoming overweight. I'm sorry to say that there is no solution to this issue if you are a shift worker; I can only advise that you make sure that you still get seven to eight hours of unbroken sleep, and in a darkened room.

So once you have spent the evening relaxing for at least three hours - it’s time to go to bed to luxuriate in those seven to eight hours of blissful, fat-burning sleep.


Karatsoreos et al (2011). ‘Disruption of circadian clocks has ramifications for metabolism, brain, and behavior’, Proc.Nat. Acad. Sci. 108(4): 1657-1662

Roenneberg et al (2012). 'Social jetlag and obesity', Current Biology 22(10): 939-943

Knutson K. L., & Van Cauter E. (2008). ‘Associations between sleep loss and increased risk of obesity and diabetes’, Ann. New York Acad of Sci. 1129(1): 287-304

Beccuti G. & Pannain S. (2011).‘Sleep and obesity’, Curr Op Clin Nut and Metab Care 14(4): 4

© 2016 Maria Cross


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