Sleep experts have known about the relationship between melatonin and light exposure for a long time. In fact, both melatonin and light are said to control the synchronization of the biological clock.
Melatonin, the chemical that is known to be important to sleep, can be controlled by light. In fact, light exposure suppresses melatonin production. This is why it's hard to sleep in a room that isn't dark.
If you are sleepy during the day, sleep experts suggest going outside and exposing yourself to large doses of sunlight during the middle of the day instead of taking a nap. This will suppress melatonin production and give you a "wakeup."