A reader asks:

I was wondering about taking melatonin. I suffer from SAD and have just started light therapy with great success. I am still drowsy in the afternoon and have read about taking melatonin in the early evening. Does this work?

Answer:

Not for relieving afternoon and evening slumping. Melatonin taken at a high dose (for example, 2 mg) during during waking hours can promote sleepiness. At a low dose (for example, 0.2 mg), afternoon and early evening administration does not directly make you sleepy, but it does nudge your internal circadian rhythm earlier, so you are likely to be ready for bed sooner than otherwise. Melatonin is not activating in either scenario. Warning: One should never take melatonin at the same time as light therapy, for two major reasons. (a) Melatonin sensitizes the retina to light, and we must avoid that risk to the eyes. (b) The action of melatonin and light on the internal clock conflicts: one will nudge the clock earlier while the other nudges it later, depending on the time of day of administration.