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: wild hazel dormice average about three years, with the longest recorded wild lives running to four or five, against barely a year for a wood 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.