In general, ferrets sleep quite a bit, even adults.  A two- to four-
hour playtime followed by a several-hour nap is typical.  Ferrets
sometimes appear to be sleeping with their eyes partly open, and they
sleep very heavily, often not waking even when picked up.  You can
take advantage of this and try to cut their nails while they're
asleep.  It means you have to be especially careful where you walk and
sit, though.