Ferrets sleep a lot. They need it for health and well being. The dark is important for them, as it regulates their body temperature, and they need to eat and poop while asleep too.
They like to sleep in dark places that are sheltered. They are burrowing animals, so sleeping under things is natural for them. They also want to feel secure and snuggled up so groups of ferrets can sleep together for warmth. They need a place to sleep that is safe from people and other pets who may hurt them. This is why a cage with bedding on the second or third level is ideal for them. This allows the ferrets to eat, drink and toilet on the first level and have a safe bed on the higher levels.
When a ferret is sleeping, it is hard to wake them up. They are very deep sleepers, and they snooze through the day and night. During the winter this is even more true as they need to conserve energy for the colder weather.
Sometimes a ferret will get so deep into its sleep that it looks dead. This can be upsetting to new owners who panic thinking their pet is dying. However, if the ferret is warm with a moist pink mouth and breathing regularly but slowly, it is just asleep, and not dying or in a coma.