When I take a power nap I wake up more tired and very nauseous. My head feels heavy particularly on the left side. What could be causing this?

Naps and grogginess. If people go into slow wave or deep sleep during a nap, and then wake up from deep sleep, they may feel groggy and tired, and their heads might feel "full". This can happen if the nap is long, or if the person is sleep deprived (and therefore goes into deep sleep quickly). Make sure you are getting 7-8 hours of sleep a day, and keep your naps short.