Some people get the winter blues on a monumental scale. Sufferers of seasonal affective disorder (SAD) would rather cosy up to a television set than another human being. They shun sex in favour of pizza, snooze for maybe 16 hours a day and are often moody.
Small wonder that SAD sufferers compare their condition to hibernation. When hibernating animals prepare for winter, their metabolism slows down, their sex drive wanes and they sleep in dens, surviving till spring on fat reserves.
The theory that seasonal depression is an atavistic form of hibernation was once dismissed by researchers. But recent studies have reawakened interest in the theory.
It’s true there are major differences between seasonal blues and other forms of depression. Clinically depressed people usually lose interest in food, finding it tasteless or even unpleasant. They often shed weight and have great trouble sleeping. Sufferers of seasonal depression are just the opposite, eating and sleeping with gusto.
While SAD affects just a certain percent of the population, many researchers believe that most of us are susceptible to seasonal overeating, oversleeping and a general bodily go-slow. Some even say it’s the extreme end of a spectrum of adaptive responses to winter weather.
‘We want to establish whether SAD is part of our genetic background,’ says George Wilson of the University of Tasmania in Hobart. ‘It could be a programmed reaction to shorter daylight hours in winter.’
Recent studies have uncovered hibernationlike physiology in people with SAD.
Margaret Austen – one of Wilson’s colleagues in Hobart, where winter nights average 15 hours long – looked at SAD-related changes in the autonomic nervous system. These nerves regulate functions such as breathing and heart rate and are involved in hibernation. They could be just as prominent in seasonal depression, too.
There are two parts to the autonomic nervous system which work in opposition to control bodily functions. The ‘sympathetic’ system boosts metabolism, while the ‘parasympathetic’ system damps down bodily functions.
Just before animals hibernate, they experience a spike in the activity of their parasympathetic nervous system, which slows their heart rate and decreases their body temperature and metabolic rate. Austen found a similar parasympathetic response in people with SAD.
‘Animals prepare for winter by fattening up and then sleeping through it,’ says Austen. ‘In humans that is not practical. So, instead, we eat more and gain weight throughout the winter, and as a result… lack energy and sleep more.’
A similar study in Russia found that hibernation-like activity in SAD patients (binge eating and excessive sleeping) are signs of an adaptive mechanism aimed at conservation of energy.
A fizzling winter sex drive is an adaptation to the winter chill, too, says Thomas Wehr of the National Institute of Mental Health near Washington DC.
Evidence shows that human responses to seasonal changes may have been more pronounced before electric lighting was common.
Low sex drive in winter could have served both to conserve energy through the winter and ensure that your offspring are born at a time when food is available.
Babies conceived in winter would be born in autumn, when food is starting to become scarce. Babies conceived in summer would be born in spring, when food is starting to be plentiful.
Matthew Andrews, from the University of Minnesota at Duluth, discovered that a number of genes used by animals to kick-start hibernation in the way they convert fat reserves are also found in humans.
So is SAD an evolutionary leftover? Andrews says that the existence of similar genes alone is not necessarily proof that humans once hibernated. Still, he adds: ‘If there were any vestige of hibernation in humans, it makes sense that it would be something like SAD.’
Day length is always a key factor in hibernating animals. The shortening period of light tells the body’s internal clock that winter is approaching.
Shorter days also trigger SAD. Treatment usually includes daily sessions in front of a strong light source. This phototherapy works by tricking the circadian pacemaker in the brain, improving mood and reducing lethargy.
Wehr believes that all humans have the potential to succumb to seasonal alterations, but that most of us can ignore changes in day length because we live in a world of artificial lights.
‘I suspect what we call winter depression has its origins in evolutionary biology,’ he says.
‘The symptoms might well have been normal behaviour, but now we view them as extreme.’
Despite the many biological similarities between hibernation and ‘seasonality’ in humans, many researchers are far from convinced.
Hibernating squirrels can drop their body temperatures to just above freezing for weeks at a time. Even bears, much closer to us in size, are capable of surviving up to five months on their own body fat, neither of which we could do.
But lots of non-hibernating animals make it through winter in a torpid state, reducing their body temperatures and whittling their metabolisms down to a minimum.
Since humans evolved out of and left the equatorial climes of Africa, perhaps our hunter-gatherer ancestors that ventured further north may have evolved a similar ability to survive long periods without food.
Nowadays, we just have to survive long nights. Luckily, we can crawl under the covers, get the candles lit and the fire roaring and hit the remote control. Don’t feel guilty. After all, you are just doing what comes naturally. SOURCE