dremati

1. to doze; to nap
   Deda drema u fotelji posle ručka. — Grandpa dozes in the armchair after lunch.
   U autobusu uvek dremam do posla. — I always doze on the bus on the way to work.
2. colloq. to be asleep at the wheel; to not pay attention
   Ne dremaj, svetlo je zeleno! — Wake up, the light's green!

Unlike spavati, dremati means light, short sleep.