Casting Sleep on a mounted character

[Talakeal]

Quick question I am hoping someone with a better grasp of the mechanics of horseback riding can answer. How would you resolve a sleep spell being cast upon a mounted combatant? This came up during a recent game and I didn't know how to arbitrate it.

Do they just slump over held in place by the saddle?
Do they fall of and injure them self?
Are they awakened by the act of falling itself?

Thank you for any advice you can give.

[Grumman]

This is mentioned in the Skills and Equipment sections of the PHB

If you're knocked unconscious while riding, you have a 50% chance of staying in the saddle, 75% if it's a military saddle, or 100% if it's a burrowing saddle.

If you fall off, you make a DC 15 ride check or take 1d6 damage. If you're asleep, you probably automatically fail this check.

Wounding a sleeping creature wakes them up.

[Starscream]

Personally I think it would be more fun to cast it on the mount.

[herrhauptmann]

Well it's an area effect, so no reason why you wouldn't also get the horse. Though in low level games, I believe the horse has more HD than the rider, so he'd be taken out last by the spell. (Weakest creatures go first, same with turning)