Kitten is always sleepy....?

my kitten is about 2 and a half months already and recently i just started letting him roam outside because he started to just sit in front of our sliding door as if he really wanted to go out. last week i started letting him out about an hour a day, hell go to our neighbors and to the side of our house and hell always be back like in an hour, i started realizing that hes been sleeping alot more now ever since i started letting him out. he eats and drinks like normal hes just always sleeping. should i be worried?