Flower buds are starting to swell, migrating birds are heading north, and in March we move our clocks forward to capture an extra hour of evening light as the days grow longer. Our bodies track the seasons by monitoring daylength. That exposure to light interfaces with our circadian clock, and can influence our moods, our metabolism, and even our memory. Tennyson (1809-1892) understood this well, writing ‘In the spring a young man’s fancy lightly turns to thoughts of love’.
But how, exactly, does the length of the day get measured? While the circadian clock oscillates in a 24-hour cycle, the seasons are on a much longer time scale. As the days grow longer, the ambient light coincides with different phases of the circadian cycle. The coordination (or the offset) of daylight with the circadian clock determines the biological response in both animals and plants. The combination of light inputs and clock factors shape the seasonal expression patterns of key genes by affecting transcription, protein stability, and chromatin structure. Interestingly, the signature of human migration from equatorial climes to other latitudes is reflected in the selection for particular variants in core circadian clock genes (Forni et al, Genome Biol. 2014). Thus, while we all look forward to more hours of sunlight, some of us may be genetically fine-tuned to appreciate the lengthening days all the more.
The Plum Blossom Garden at Kameido, by Utagawa Hiroshige, 1857.