are you a heavy dreamer at all? I recently read that having a lot of dreams makes you feel tired mentally and affects brain chemistry. This happens to me sometimes if I oversleep and I wake up tired with a headache. It could also be mild depression, bad sleep schedule, lack of good health/exercise, no motivation, busting too much (losing energy), etc.
what I found earlier this year : Vivid Dreams cause of excessive sleepiness - Undiagnosed Symptoms - MedHelp
what I found earlier this year : Vivid Dreams cause of excessive sleepiness - Undiagnosed Symptoms - MedHelp