How Not enough Sleep = Weight Gain

Getting a few hours less sleep than  you need raises a hormone called Ghrelin. This is the hormone that tells you when to start eating in the morning.   So, the more Ghrelin you have, the more breakfast you will eat.  Less sleep also SUPPRESSES leptin. Leptin tells you when you are full and can stop eating.     

What this means is that when you are TIRED, you will eat more and will think you are hungry.   

This is ACTUAL body chemistry and it is hard to fight that. I know for me, when I am tired I ALSO go for MORE caffeine (which raises cortisol levels), AND I turn to more sugar for that pick me up energy high (followed by a crash)  Of course, sugar spikes insulin levels, causing your body to pack it away quickly as fat.

Getting less sleep also stresses your body. When you are stressed, your body increases it's Cortisol , ALSO leading to extra fat storage and a lower metabolism.

Ancient wisdom, although absent of the 'science' was right.  A good night's rest will do you some good.... ;)