We really tried to sleep below passes so we tackled them when we were fresh from sleep. It won't always be possible with a 14 day plan, like we had, and you may even do two passes in one day.
In the end we actually did a few of them in the evening, and were even hiking on the backside of them at dusk, and even dark. It sounds good, in theory to do them in the morning, but it just seemed like it was better "to just get it overwith" some days.
We were on a strict schedule, with stays lined up at Red's and MTR, and with friends meeting us below Baxter Pass. So we didn't have the luxury to slow down when we were tired. The first few days were really hard, I was not prepared for it. By about the eight day mark, my endurance was fine, but our feet were beginning to really take a toll, you just end up living with sore feet if you don't prepare them. I had the gear and logistics extremely figured out, but not the training.