It's not for certain whether DP is caused by too much serotonin -- or perhaps, too little dopamine. But the fact that your DP changes at night could be a sign of something else which may help you find the source of your DP.
You could be right about the circadian cycles and if that is the case, you should consider a sleep study in order to see what changes occur within your brain while you sleep.
There is definitely a strong link between disrupted sleep cycles and short-term DP. However, for long term DP the case may be a prolonged sleeping disorder which has been going on for years!
Whatever the case may be, go see your doctor. Another user pointed out S.A.D but you didn't mention you have any mood disorders so I"m not sure whether that is the case.
