Ideally it is best to increase levels naturally with exposure to bright sunlight in the daytime (along with full spectrum fluorescent bulbs in the winter) and absolute complete darkness at night.
If that fails or isn’t possible, you may want to consider a melatonin supplement.
In scientific studies, melatonin has been shown to increase sleepiness, help you fall asleep more quickly and stay asleep, decrease restlessness and reverse daytime fatigue.
Melatonin is a completely natural substance, made by your body, and has many health benefits in addition to sleep. Start with as little as 0.25 milligrams (mg) and work your way up in quarter-gram increments until you get the desired effect.1Sleep — Why You Need It and 50 Ways to Improve It