First, during deep sleep, your brain secretes a large amount of a growth hormone, which tells your body how to break down food for fuel. Second, you probably eat more when you are tired. Therefore, without adequate sleep, you eat more and your body is incapable of breaking down the fat, thus weight gain happens.
What can you do? You can start by going to sleep and waking up at the same time every day. Watch your caffeine and alcohol habits - no caffeine after 2:30 pm and no alcohol three hours before bed. Also start a bedtime routine. Create a pre-sleep ritual such as light reading, a hot bath, or stretching-beginning about 45 minutes before lights out.
Remember to experiment with exactly how much sleep you really need. Increasing your sleep to what your body really needs not only gives you a thinner waistline, but also more energy!