Using colour to tell time

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How do animals tell time of day? This is an important question because internal circadian clocks, which drive daily rhythms in practically every aspect of physiology and behaviour, must be aligned with external time. For many years its been accepted that the most important external signal is the huge day:night difference in ambient illumination. There is a great deal of evidence that the retina sends information about environmental brightness to the circadian clock in a part of the brain called the suprachiasmatic nuclei (SCN) and that this signal can reset clock phase. However, we have recently shown that this is not the only way in which light can be used to tell time. Across twilight there is a big change in the spectral composition of natural light with shorter wavelengths enriched at low solar angles. As a result, light appears progressively bluer as solar angle decreases. We can perceive this change in colour ourselves and, in fact, because it is relatively unaffected by clouds, it provides a more reliable indicator of time across dawn and dusk transitions than does brightness. Now, using a range of sophisticated visual stimuli, we have shown that the mammalian SCN is sensitive to changes in colour and that including changes in colour improves the phase of entrainment of mice exposed to naturalistic light cycles. Those data represent the first demonstration that mammals can use colour to tell time. We now would like to know how the SCN comes to be colour sensitive; how the clock's response to light is defined by its intensity and colour; and whether the clock of diurnal animals is also colour sensitive. Answering these questions will provide basic information about this newly discovered component of sensory biology. It also represents the key knowledge that we need in order to define the practical importance of our discovery. Disruptions to entrainment of the clock (e.g. in shift-work, ageing and various diseases) in both humans and animals are associated with range of negative effects on mental/physical performance, health and well-being. There is already substantial interest in how modern visual displays and artificial lights affect the clock. On the one hand, there is concern that too much light at night and/or too little natural light during the day (as e.g. for indoor reared animals) impact clock phase. On the other, there is excitement in the possibility that by changing the characteristics of light one could design systems that better support natural rhythms. Until now that effort has concentrated on the impact of different light intensities. Its colour has been considered but only insofar as some wavelengths more effectively induce a 'brightness' response in the clock. Our data also argue that we should think about its true colour. The experiments described here represent the essential steps in determining whether and how we can modulate colour to support appropriate circadian physiology in man and domestic animals.