As smokers, we were accustomed to receiving doses of nicotine and approximately 4,000 other chemicals 20 to 40 times a day. The stress of abruptly cutting off that supply, as unhealthy as it was, can leave us feeling extraordinarily fatigued.
If you're tired and can manage it during the day, take a power nap, and go to bed a little earlier than usual if you need to. Your body is working hard right now to overcome the effects of nicotine withdrawal, and some extra sleep will do you good.
On the other hand, if you're at the opposite end of the spectrum and find yourself unable to sleep, which is also a common side effect of nicotine withdrawal, try taking a long walk several hours before bed.