Sleep indicates a failure of mindfulness; mindfulness must be maintained, even in absorption.
The 5 jhana factors:
1. Bringing the mind to the object (arousing, applying)
2. Keeping the mind with the object (sustaining, stretching)
3. Finding, having interest in the object (joy)
4. Being happy and content with the object (happiness)
5. Unifying the mind with the object (fixing).
"The final stage will be the sinking in, absorption or unification with the object. This is one-pointedness. Again there must be some mindfulness at the very quiet and still stage before absorption. Otherwise one may lapse into deep sleep or is pulled away by subtle thoughts. At this last phase, the Visuddimagga quotes joy as overcoming restlessness, bliss as overcoming ill-will, and one-pointedness as overcoming sensual desires because it stops the mind from flitting to sense objects and fixes onto the meditation object." http://www.buddhanet.net/mettab3.htm
Sloth and torpor, one of the 5 hindrances is mental, while sleep is physical, however unwise attention to torpor can lead to sleep. Each of the 5 jhana factors opposes one of the 5 hindrances.