Need to change that spare tire around your middle? Well, lack of sleep will sabotage your best efforts in a dozen different ways. Or maybe you are someone who feels like you have suddenly packed on some extra pounds inexplicably. One of the first places to look is the quality and quantity of time on your pillow.
One of the primary reasons has to do with appetite. Everyone knows how they get “the munchies” much more frequently when they are tired and there’s a reason for that: your body dramatically alters your hormones to compensate. For example, one study looked at participants with ample sleep and then after four hours of sleep. It was no surprise that they “found that sleep restriction was associated with an 18% decrease in leptin, a 28% increase in ghrelin, a 24% increase in hunger, and a 23% increase in appetite”.  Leptin and gherlin are two of your primary hunger hormones and sleep shifts both of them in the wrong direction.
This means your appetite and hunger skyrocket correspondingly and, unless you are iron-willed, you will end up eating more on little sleep. One 2009 study showed that those with 5.5 hours sleep ate 22% more in snacks compared with when they had 8.5 hours of sleep.  That’s a hefty boost in calories – over 200 on average – that will pack on the pounds faster than you can say lardbutt.
And a little extra snacking is just the beginning of your endocrinological nightmare: lack of sleep also lowers your testosteorne and growth hormone (as I document in my link on Sleep and Testosterone and Sleep and Growth Hormone). The ensuing loss of testosterone will likely eventually result in a loss of muscle and we all know “muscle burns fat”. In other words, low sleep will eventually reset your metabolism in the negative direction. This is further amplified by the loss in Growth Hormone, which is responsible for “leaning you out”.
Sleep loss will also boost your cortisol levels and cortisol is associated with the most deadly kind of fat gain: visceral fat. That’s the kind of fat that accumulates around your internal organs and is associated with heart disease.
Just as deadly is the fact that lack of sleep will also whack your blood sugar metabolism and make you insulin resistant. Of course, insulin resistance is part of the deadly (and sexually devastating) Metabolic Syndrome, but it will also make you fat. The high insulin levels are deadly because the block the breakdown of fats by adrenaline and lipase. 
Lack of sleep also makes you feel much less like exercising. Your cortisol, insulin and appetite are all increasing and your testosterone and growth hormone are decreasing, yet you don’t have the energy to help compensate. The bottom line is that burning the midnight oil is going to pack on the pounds and those pounds will lead you to extra estrogen, calcified arteries, poor mental performance and a look of bewilderment as you try to figure out how in the world you put on all those pounds.
1) Annals of Int Med, 2004, 141:846 850,”Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite”
2) Am J Clin Nutr, 2009, 89:126-133, “Sleep curtailment is accompanied by increased intake of calories from snacks”
3) Kobe J Med Sciences, 2007, 53:99-106