Yes, SLEEPER is a Scrabble word!

Letter values in Scrabble US: S 1, L 1, E 1, E 1, P 3, E 1, R 1.

Is it a valid word?

SLEEPER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a rester who is sleeping
a spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal
an unexpected achiever of success
one of the cross braces that support the rails on a railway track
a passenger car that has berths for sleeping
pajamas with feet; worn by children
a piece of furniture that can be opened up into a bed
tropical fish that resembles a goby and rests quietly on the bottom in shallow water
an unexpected hit

Also: railroad tie, sleeping car, wagon-lit, sleeper goby.