Sleepy, drowsy, somnolent, slumberous mean affected by a desire to sleep or inducing such a desire.
Sleepy , the ordinary term of this group, applies not only to persons but to things that suggest a resemblance to persons who show a readiness to fall asleep. The term also applies to conditions or to things which incline one to sleep or to dozing or dreaming.
Drowsy differs from sleepy in carrying a stronger implication of the heaviness or loginess associated with sleepiness than of the actual need of rest. When applied to things rather than to persons, drowsy connotes more obviously than sleepy a soporific power.
Somnolent may be used in place of drowsy ; usually, however, it connotes the sluggishness or inertness characteristic of one who is sleepy or drowsy or the capacity for inducing this rather than the actual impulse to sleep or doze.
Slumberous is often used in the sense of sleepy or drowsy or somnolent ; occasionally it carries a distinctive connotation in which it usually suggests quiescence or the repose of latent powers.