Yes initial tiredness I think. It's important to take extra time to wake them early enough (a full 1h15mns before having to leave home), have them something else to do before leaving home that can be part of the pre school routine, so they wake up properly... and then go. Also, re their experience of the matter, going to school leisurely is much better than going stressed (to drop them off quickly and then rush off to work)... All of this of course requires an early sleep at night, for them, and for you as well:-)