Today, the 21st of June, is the longest day of the year, commonly known as the Summer Solstice. The sun will be in the sky for 4 seconds longer than yesterday and 2 seconds longer than tomorrow. But more importantly, in the UK Summer has now officially started, and surprisingly we have blue skies and sunshine in London.
I thought I’d take this opportunity to read up on and discuss whether our body has an annual clock, as well as a daily clock.
To measure the time of year, an animal can either know it is Winter, because the daylength (exposure to light) is short, or it can have an innate clock inside that measures one year. Or it can have both.
In some species annual clocks are very apparent: in Autumn birds begin to migrate to warmer climates, in Winter bears hibernate, in Spring lambs are born and in Summer we see our garden plants flourish.
There is much less research carried out on annual rhythms as opposed to daily rhythms. It takes scientists much longer to collect the data, and there are many variables that can cause any observed rhythms, which makes the data harder to interpret.
In humans it is especially tricky. Length of daytime is not the only thing that alters during the seasons. There are potential effects of temperature, climate, food availability, agricultural cycles, and holiday seasons, to name but a few.
One study has looked at the quality of embryos and sperm produced for IVF treatment in Iranians between June 2001 - June 2004, which included over 1000 treatment cycles . They showed conception was much more likely during the Spring months of March-June. This time was also the best for quality of embryos and men had a higher sperm count. However, no changes were observed in egg number, sperm quality, and other factors.
|Seasonal effect on number of successful pregnancies following IVF treatment in different seasons, (Vahidi et al, 2004)|
Has electric lighting, and the ability to have as long or short a daylength as we like, disrupted our annual clock?
Another investigation published in 2004, examined the birth patterns in Spain between 1900-1978 . Up until the 1960s there was a strong correlation between season and birth. However, when industrialization and subsequent changing in heating and lighting conditions was introduced the amplitude of this rhythm dropped: there wasn’t such a difference observed between different seasons.
The question whether humans still have, or if we ever had, an annual clock of changing biology and behaviour is still open for a lot of research and debate. Russell Foster, a researcher at Oxford, has written an interesting and in depth book on annual body clocks, which I would recommend to anyone who enjoys this topic.
 A. Vahidi, S.M. Kalantar, M. Soleimani, M. Hossein, A. Arjmand, A. Aflatoonian, M.A. Karimzadeh, and A. Kermaninejhad, “The relationship between seasonal variability and pregnancy rates in women undergoing assisted reproductive technique,” Iranian Journal of Reproductive Medicine, vol. 2, 2004, p. 82–86.
 T. Roenneberg, “The decline in human seasonality.,” Journal of biological rhythms, vol. 19, Jun. 2004, pp. 193-5; discussion 196-7.