Winter is here – almost.
The autumn equinox this year was 22 September. From then on the sun peaks at midday overhead somewhere in the southern hemisphere until the spring equinox late next March. In practice, day and night are of equal length on the equinox, or would be if our earth fortunately didn’t have an atmosphere. In London, on 21 June the sun rises at 4.43 and sets at 21.21; on 21 December the corresponding times are 8.03 and 15.53. That’s nearly nine hours more light in the summer. With such a dramatic difference at this latitude, no wonder so many react to the difference.
Light plays a central role in regulating our biological clock. We live on a natural rhythm called the circadian rhythm, and our internal clock is set by the action of light on the retina of the eye, transmitted by special tracts of nerve cells to the pineal gland. The pineal gland, situated near the centre of the brain, manufactures a hormone called melatonin. Melatonin helps send us to sleep, so much so that in some counties it is available without prescription as a sleep aid, and melatonin is also used in overcoming jet lag.
Light then is essential for keeping us awake, and sleep plays an important role in maintaining our mood. Most people have heard of seasonal affect disorder (SAD). The definitive mental illness diagnostic system, the American Psychiatric association Diagnostic and Statistical Manual of Mental Disorders (DSM, now in its fifth edition) officially classifies seasonal effects on mood as “recurrent major depressive disorder with a seasonal pattern”. In winter people with SAD are – well, sad. Some people become depressed only in the winter months, and maybe autumn and spring too, and for some people their pre-existing depression becomes worse. Unsurprisingly, given that in winter days are shorter the further north (and south in the southern hemisphere) you go, there are substantial geographical variations in the incidence of SAD, In the USA, in Florida the figure is very low, just over 1%, and in Alaska nearly 10% of the population is affected. Pity the three hundred thousand inhabitants of Murmansk, situated north of the Arctic Circle, which does not see the sun at all between 2 December and 10 January.
I should point out that not ever researcher accepts the existence of SAD. Some studies have failed to find any correlation between mood and time of the year. As with all studies on this sort of subject, much depends on the detail of exactly how mood is measured, how many people are studied, and whether or not they are receiving any treatment.
If people’s moods are affected by the amount of sunlight available then you would expect the suicide rate to vary with the seasons. It does, but not in the simple way you might expect. In the northern hemisphere the suicide rate increases dramatically in May and to a lesser extent June, and in the southern hemisphere in November. This pattern is strange and there is no accepted account of why it happens. One explanation is that when people are very depressed they are too ill to kill themselves, and need the upsurge in energy when they are starting to feel better. I don’t find this explanation wholly satisfactory because I have always felt most suicidal when I feel most depressed; it’s then that I want the pain to end. Most people when they start to recover feel relief. Another possibility is that when people are improving there is a surge in the chemical, or neurotransmitter, serotonin in the brain, and serotonin is associated with aggression as well as mood. In depressed people aggression can be directed towards themselves, leading to self-harm and suicide.
Suicide rates also vary across regions. If you look at a map of Europe there is an increase as you go from the south and west to the north and east, and again it is not simply the case that suicide is always more common in cold, dark regions; socio-economic and cultural factors play a large role too,
I graph my own mood, as I suggest everyone with a depressive disorder does, and have noticed a slight seasonality effect, but it is much less pronounced now I that I am on fairly effective medication.
We are not completely helpless when the nights start drawing in. Those fortunate to be able to overwinter in southern California should now start thinking about packing their bags. Those a bit less fortunate should book their winter holidays, going somewhere likely to have as much sunshine as possible. For the rest of us, there are still things we can do. SAD lights, which emit very bright light (look for at least 10,000 lux) and which produce light in the shorter, bluer frequency range, are now cheaper and much more widely available than they were just a few years ago. But one of the best therapies is free: being outside in natural light as much as possible, particularly in the morning, especially if it’s sunny. Wrap up and get outside.
(The above is an expanded version of my new column in What’s hot London!)