The difference between a physical sleep spell and a mental sleep spell is the difference between "I don't know why I'm so tired, but I need to sleep now" and "I'm not tired, but, yes, you're right, I shall sleep now".
If the command, instead of being to sleep, was to jump off a cliff to the victim's inevitable death (quite clearly a physical result), would you still have the incremental effects be physical?
What if the command was to follow verbal instructions which included jumping off that same cliff?
What if the command was to take no actions, despite the physical assault the victim is being subjected to by the mage's allies?
What if, instead of a command, the spell inflicts crushing apathy resulting in the target losing the will to so much as breathe?
The end result in each case is clearly physical, after all.
And yet, Incite Emotion would suggest that the last, at the least, is definitively a source of mental stress and consequences.