Usually mid afternoon, even ten minutes can rejuvenate. If only more employers realized that employee productivity and work quality would be improved with a 30 min siesta.
I can grab a nap just about anywhere. When I was traveling by air a lot I could fall asleep during take off. When driving a car while sleepy (very dangerous) I pull over, recline the seat and zonk for just a couple of minutes. Then I'm good for a couple more hours.