In my experience, just starting to doze off is enough for a refresher. This may take all of 20-30 minutes in itself, I can often go to sleep at night faster than I can fall asleep during the day. Salvatore Dali was famously said to have held a spoon in his hand suspended above a metal pan and when he fell asleep the spoon would drop and wake him with its sound. Just lying down for a period with eyes shut and little external stimulus is a great refresher for me. Finding the time/place to do this is the hardest part.