Lin (Sydney, Australia) on 02/06/2013:

Depressed people are usually in the REM sleep state for a lot of the night (compared to non depressed) and hence wake feeling exhausted mentally and with low levels of serotonin.

Throughout the day activity gradually increases the serotonin levels but of course the cyle begins again each evening.

Placebo trials have also shown that increasing the level of serotoninvia medication does little to affect the nervous anxiety and exhaustion created by over REMING.

So basically the reduced serotonin experienced by depressives is a symptom rather than a cause.