Ovinomancer
No flips for you!
Some element of the action (or movement) that begins combat wakes them up. Once they are no longer asleep, they no longer have the condition.
This line of reasoning runs counter to my experience of sleep and awareness. When I'm asleep, although my eyes are nearly always shut, my ears do not close up. They pick up any noise around me just as if I had been awake and send information to my brain about those noises. But I'm aware of none of this. If you asked me later what sort of noises I heard during the night I wouldn't be able to tell you because I would have no recollection of having heard them. My subconscious brain is keeping track, however, and if it picks up anything it considers alarming, it wakes me up, and only then do I become aware of what's going on around me.
The spell explicitly imposes magical sleep. Combat has already started when creatures are affected, however, because the casting of the spell itself triggers combat if it hasn't started already. I would attribute that the sleep can only be ended for the spell's duration by the enchanted creatures taking damage or being shaken or slapped awake to the sleep's magical nature, but I would certainly allow either of those events to awaken those affected by mundane sleep as well.
My issue with this is the fluctuation between 'unconscious and cannot perceive anything' and 'something in the environment wakes them up'. Presumably, they perceive this something? The inconsistency here is one of expectation: can I rely on the game world to perform as expected. To expand with an example, if an NPC thief sneaks into a sleeping PC's room, according to your previous points the thief can burgle to his hearts content and not wake the PC. But, if the PC discovers the thief, and sneaks into the thief's lair while the thief is asleep, and then attacks the thief in his sleep, suddenly, despite any training or care, the PC makes a noise, or flashes moonlight, or does something that awakens the thief prior to the attack. The double standard here would be jarring to me. You can steal or do anything else without a check or chance of awakening a victim so long as you do not attack. Attacking automatically means that the sleeping target awakens fully.