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; 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.