My husband sleeps for 15-18 hours a day according to my 17 yr old son who lives with him what could cause this extreme level of sleepiness?

Not usual....: Obviously, this duration of sleep is not usual. Depression can sometimes make people sleep all the time. So can profound hypothyroidism. I wonder what his energy level is - and how he feels in general. He clearly needs to be evaluated. Good luck. Lgromkomd.

Need to see a sleep : Need to see you pcp and if needed he will refer to a sleep lab.