Causes of excessive sleepiness are sleep-wake cycle disorder like in night shifts, sleep apnea and other sleep-related breathing disorders, psychiatric disorders like depression and schizophrenia,diabetes,obesity,restless leg syndrome,use of drugs or alcohol and narcolepsy(Abnormal sleep disorder leading to excessive daytime drowsiness). In your case it can be due to sleep-wake cycle disorder.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.