Is going to bed too late making you fat?

Many of our hormones are intimately related to circadian rhythms, and their function gets thrown off when we completely ignore the fact that night follows day, and day follows night. During the longer days of summer, mammals are hardwired to find sustenance to store as fat, to help them last the shorter days of winter where they will be either hibernating or eating less due to lack of food availability.

In humans, this hardwiring shows up as a desire for carbohydrates. By choosing to stay up with the lights on, watching TV or doing computer work late at night long after the sun has gone down, our hormonal system is fooled into believing that it is still day, resulting in increased cortisol and insulin levels, which makes you want to snack, and most likely on carbohydrates. In today’s wealthy countries, winter does not mean a lack of food availability, so late night snacking is often the result.

Even if you do not snack, staying up late regularly causes cortisol to release a nightly dose of sugar into the bloodstream for energy, insulin then rises to store that sugar as fat. Constantly high levels of insulin are an important factor in developing insulin resistance and type 2 diabetes. And if you are insulin resistant, smelling a pastry will make you fat! Therefore, fat storage is largely controlled by the amount of light you are exposed to and the insulin resistance it causes. (note: protein types will often sleep better with a small protein snack before bed.)

Going to bed by 10pm lowers your night-time cortisol and insulin levels, and raises your melatonin levels, which prevents this whole cycle from developing. In the summer we can get away with staying up later, but when the sun goes down, bed should soon follow. In the winter most of us need about 9.5 hours of sleep.

Chek, Paul; How to Eat, Move and Be Healthy! Chek Institute, San Diego, CA, 2004.

Chek, Paul; Oliver, Clifford, Remsen, Julie; Optimum Health and Fitness Through Practical Nutrition and Lifestyle Coaching Chek Institute, San Diego, CA, 2002.

Formby and Wiley; Lights Out! Sugar, Sleep and Survival Pocket Books, New York, NY, 2000

Smolensky and Lamberg; The Body Clock Guide to Better Health Holt and Company, New York, NY, 2001.


  1. vanes said,

    May 17, 2007 @ 10:38 pm

    Helo Good Day!!

    im 21 yrs. old already but my body is too tin..Im eat more but
    im not going fat..what i shall do?
    hope you can help me…
    God Bless..

  2. Vreni said,

    May 20, 2007 @ 3:15 pm

    Hi Vanes,

    I would suggest you go to your doctor and rule out type 1 diabetes or any other problem that may prevent you from gaining weight.

    Otherwise, eat nutrient-dense, high quality food. Eat good quality fat and protein, as that is what builds your body. See my nutrition tips for help in what foods to eat and what to avoid.

    All the best,


  3. Julie said,

    January 26, 2009 @ 6:31 pm

    What if your job/classes force you to stay up late and not get much sleep? Would it be helpful to work in an environment with dimmed lighting?

  4. prithwijit mukherjee said,

    July 24, 2011 @ 2:47 am

    what should be the appropriate time to go sleep??…. i have to get up at 545 am everyday….and i genrally get tired as i go to gym after coleg…..and still i stay up late til…. 1am usually…..on the contrary i eat too much i think….and my weight is fixed at 63 kg….am 21 years old….is there any reason for concern?

  5. Vreni said,

    July 24, 2011 @ 11:11 am

    You may be fine now but at some point this lifestyle will catch up to you. I know because when I was in my 20s I did the same thing. I’d strongly recommend you make an effort to get to bed by 9h30 or 10 so you will get at least 8 hours sleep. If you can get into the habit of going to bed early now, you will remain far healthier when you are older. Very hard habit to develop though, so best of luck to you!

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