Sleep!

"Sleep accomplishes a lot of important things, metabolically
and hormonally. When you disrupt the natural rhythms of sleep,
you play havoc with many important biological processes such
as the replenishment of biochemicals and the release of important
hormones (like melatonin and human growth hormone). And recent
studies show that even a small amount of light during sleep throws
off your natural circadian rhythms, disrupting appetite, and
potentially leading to weight gain. Try to sleep between 6-9 hours
a night, uninterrupted, in the dark."

-Amazing Wellness Magazine Winter 2011