"Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate. It is the animal's slowed metabolic rate which leads to a reduction in body temperature and not the other way around.
Hibernation may last several days or weeks depending on species, ambient temperature, time of year, and fur on the animal's body. The typical winter season for a hibernator is characterized by periods of hibernation interrupted by sporadic euthermic arousals wherein body temperature is restored to typical levels"
i'd argue that most bees are not running at full whack whilst clustered but have periods of interspersed activity. sounds a lot like hibernation to me by the definition above (but not like the blue peter tortoise - which of course slept all winter but had the prospect of having it's eyes cleaned with cotton wool dipped in janet ellis' breast milk on waking).