The time of year could have dramatic effect on how people behave. In one example, hospital admissions for mania in bipolar I disorder peaked in the spring and autumn (the opposite of admissions for depression).
Suicidal behaviour is also under the influence of the seasons. It’s most common in the spring and summer months in the general population, but far lower in winter. Such ‘seasonality’ has more of an influence in people who committed suicide where depression was a factor. At the EPA 2015 last month, suicide was a topic high on the agenda, particularly with reference to mixed states in bipolar I in which there was a 54% etime risk of attempted suicide.1 (You can read our report here and more about suicide in bipolar I patients here.)
The influence of seasonality on suicidal behaviour, Professor Gonda added, also becomes stronger the farther from the equator people live. But what could be the cause?
One theory is that seasonal changes in day length (the ‘photoperiod’) cause seasonal changes in people’s brain chemistry – especially those with affective disorders like bipolar I disorder. The further people live from the equator, the more day length changes from season to season – and the greater the effect of seasonality on certain behaviours. For example, the spring peak in suicides has been linked to the rapid increase in the photoperiod seen at this time of year.
There is a strong association between photoperiod length and levels of the neurotransmitter serotonin in the brain. Serotonin has a strong association with affective disorders. Indeed, seasonal serotonin fluctuations are especially great in people with a mutation in a serotonin transporter gene – a mutation strongly correlated to people with affective disorders.
Professior Gonda concluded with a bold theory – that seasonal changes in the brain chemistry of the general population evolved as a response to seasonal changes in our environment. Affective disorders like bipolar I may have also evolved when this normal response to seasonal changes became disrupted. Bearing such a theory in mind for future research, the speaker concluded, may lead to new approaches that help shed light on the understanding and treatment of this complex set of disorders.