Table 2.

Similarities between the hibernation response in homeothermic mammals and seasonal changes in physiological parameters relevant to sleep and weight regulation in humans.

Seasonal changes in winter months vs summer months	Hibernating homeothermic mammals	Humans
Melatonin	⇓ pineal melatonin over time of hibernation. It might be of importance for duration of hibernation (45)	⇑ length of nocturnal melatonin secretion during winter compared to summer (effect larger in men than women) (46)
Leptin	Transient leptin resistance in November (47)	No changes in summer and winter (48)
Catecholamines	⇓ α1 receptor and ⇑ in α2 receptors during hibernation (49)	⇑ cortisol metabolites during summer (50)
Haptoglobulin	⇑ in plasma during hibernation (51)	⇑ in obese subjects ⇑ in subjects with depression it correlates with insomnia and other vegetative symptoms (52)
State of vigilance	Altered	Conditioning of healthy individuals to extended dark exposure results in biphasic sleep pattern with two main sleep episodes separated by a waking period of 1–3 h (56)
Metabolic rate	Decreased	⇑ energy expenditure in winter (11.5%) vs summer (7.5% p=0.05) after cold-exposure (53)
Food intake	⇓ during the winter	⇑ during the winter due to social and ritual (Holydays Season) eating (43, 44)
Physical activity	⇓ during the winter	⇓ during the winter (53)