Ferrets just seem to be itchy little critters, and a certain amount of
scratching is normal.  Even waking up from deep sleep for a
"scratching emergency" is normal.  However, itching can also be a sign
of several problems.

If it's fleas, you'll probably see fleas or "flea dust" (bits of dried
blood) if you look closely [10.9].  Other possibilities include mites,
bacterial or fungal infections, dry skin, allergies to food or
cleaning supplies, or poor nutrition.  Excessive itching can be a sign
of serious illness, including adrenal disease [1.1], so see a vet if
you're at all concerned.  In some cases, an appropriate dose of
children's Benadryl can help an itchy ferret, but please use this only
under the supervision of a qualified vet.