I feel weak even after enough sleep.

Hello, i have been facing this problem recently since few months, I feel like I'm not active whole day. Nowadays even sleeping for 7hrs seems not enough. I have to wake up early at 6am everyday to walk my dog and then go for a gym session. I have tried different sleep hours but still i feel weak and tired whole day. I tried sleeping 6hrs, 7hrs, 8hrs, 4hrs, 5hrs,6hrs but still feel weak and tired whole day. I am worried because during college days, i could be fully active and charged up even after only 4hrs of sleep. I never felt dull in day durig College times with full day lectures and extra curricular activities. But now it seems even 8hrs sleep is not enough. I can feel my mind geting tired at end of the day. Please tell me which type of doctor specialist to consult or why is this happening?