lethargic

drowsy

dull

deficient in alertness or activity
SYN - unenergetic
"bullfrogs became lethargic with the first cold nights"

"he was feeling rather lethargic yet knew that he had a lot to do in the next few days so overcame his lassitude and got shit done."

"He couldn't stand being around such lethargic people all the time, which is why he quit hanging out with the potheads and sought a different sort of enlightenment."

"This type of summer rain always had a lethargic effect on the students."