A note on the melatonin/sleep cycle discussion:
I have not yet experimented with melatonin because I have experienced success in regulating my sleep cycles with mineral supplementation. A lack of magnesium, in particular, can affect one's ability to fall asleep and stay asleep. The calcium, magnesium and potassium triad work to govern muscular function--calcium helps muscles contract and magnesium helps muscles relax. Too much of one will inhibit the function of the others, and sometimes insomnia or low-level anxiety can be traced back to a lack of magnesium. Chronic health problems, such as diabetes or athsma, have been correlated with deficiences in this mineral triad, and many doctors are encouraging regular testing for deficiency.
When I travel on an airplane, a difficult environment for me to relax in, I take a bolus dose of magnesium, which usually allows for some sleep, and it prevents that painful stiffness than can result from extended plane flights. Effective dosage levels range from 250-1,000 mg, depending on the individual. No real contraindications, and the toxic dose is impossibily high (8,000 I think) but it's best to know the state of your mineral levels in general to begin with.