Red European squirrels (sciurus vulgaris) do not hibernate. Agile and fast that they must be, they cannot afford to build fat reserves that make them clumsy, they then could not escape their enemies like especially pine martens, birds of prey, cats, crows... They can not eat for one or two days at max, and I had them missing for as long in the past ten days, but on the third day at the latest they must eat. They reduce their metabolism a bit and their acitvity, leave the nest only for as long as is needed to reach one of their depots, get one ore two nuts, and then escape back into their drey.
Thats why they are in so deep troubles if they have no sufficient depots. They get so weak then quickly that parasites and diseases can kill them much easier due to much reduced resistence strength of theirs.
Considering their high body temperature in normal season (~40°C) and the tiny body of theirs, they have quite high energy demands. They may reduce it a bit in winter rest, but its still high, also due to their very demanding way of moving. The squirrel's walking is very energy-intensive.
Thats why they sleep once or twice over the day even in spring at summer, otsly later morning or near noon. Its not so much heat and sunshine, its because they must regenerate.