Hazel dormice hibernate in tightly woven nests at ground level, often under leaf litter or moss where temperature is stable, from roughly October until April or May. Ground level matters: it is more thermally buffered than the shrub layer where the animal otherwise lives.
Hibernation is not the whole story. During the active season a dormouse that cannot find enough food, or faces cold wet weather, enters daily torpor and lowers its metabolism rather than foraging at a loss. The combined effect is that a great deal of the year is spent inactive. It also helps explain longevity: hazel dormice commonly reach around five years, far more than a mouse of comparable mass.
The conservation reading is often misstated. Globally the species is Least Concern, but that global category conceals steep regional declines. British monitoring has recorded substantial long-term population loss, driven by the end of traditional coppicing and by hedgerow removal and fragmentation.