Melatonin is a naturally occurring hormone, which is released from the pineal gland to help you sleep. The release of melatonin is dependent on darkness and is your body's natural way of promoting sleep at night time. It can be attributed to us feeling a little sluggish in the winter when the days are shorter.
Melatonin release also usually occurs in a regular (circadian) rhythm, hence why you start to get tired at a similar time each evening.