My right ass cheek falls asleep sometimes, but only on street rides. The duration between go and "sleep" depends on how tight the jeans are that I'm wearing.
Putting your body in the same position for any length of time will cause stuff to fall asleep. The more compressed your position, the slower blood will move through your vascular system in that area, and the faster the numbness occurs.
Herniated disks which occur between L5/S1 will cause your foot to go numb, but it'll also cause your outer thigh to go numb. And it'll persist. And it'll cause a great deal of PAIN, not just numbness. Just went through this with my wife, she had surgery the week before last.. Look for a disc herniation chart online and it'll point you to what nerves in your extremeties are associated with which discs.
Here's something no one has asked.. Do your BOOTS fit properly? Are you perhaps tightening them too much? Have you tried a different brand? If your boots or leathers are cutting off circulation the most simple, straightforward solution is to switch brands or sizes on your gear... You shouldn't have to modify your riding style / position.
Just like riding on the street, to avoid the "Ass Cheek Numbness Which Causes Strange Dancing on Dismount" that I'm plagued with, I simply wear somewhat looser jeans and pay attention to "crotch binding" when I get on the bike....problem solved.