Is it psychological if I feel so tired I can't sleep, am not hungry and feel nausea? I'm taking pristiq (desvenlafaxine)

Not necessarily. It depends how long you have been on pristiq (desvenlafaxine). It can cause some nausea in the first 1-2 weeks, which could affect appetite. In addition, some people find that it either causes drowsiness or the opposite, difficulty sleeping. Would consult with your doctor, but some people find that if they have difficulty sleeping after starting Pristiq (desvenlafaxine) they should take it in the morning.