We know that light levels impact your sleepiness. How this works is only crudely understood. We know that the hormone melatonin plays a role, but current understand is pretty crude.

Researchers found that in mice, a specific type of cell (called a retinal ganglion cells), when shut off, eliminate the impact of light levels on sleep. It may be possible to trigger the sleep response through stimulating these cells (causing the body to go into it's own sleep cycle rather than overpowering it with medications and supplements) .

Of course, they haven't even been able to use this pathway to make mice sleepy yet, but it does show some interesting new directions for sleep medications.

More on Melatonin

During sleep a region of the brain known as the hippocampus is busy storing memories and recording what was learned during the day. Basically, the hippocampus "replays" important activities during your sleep. Researchers monitoring the activity in the hippocampus of 11 young and 11 old mice. They taught the mice a maze during the day (an important activity which led to food). The young mice's hippocampus "replayed" the event while the older mice's did ot have as strong of a "replay" effect. So aging may impact what the brain does during sleep which, in turn, impacts sleep. More on Increasing Your Memory