Illustration: Ulrika Nilsson Carlsson
Our body clocks are different – genes and hormones?
“Of the few studies undertaken, later starting times for schools have been shown to improve alertness and the mental abilities of students during their morning lessons. Ironically, whilst young adults tend to improve their performance across the day, their older teachers show a decline in performance over the same period!”Our body clocks are not all the same.
If you are alert in the mornings and go to bed early you are a ‘lark’, but if you hate mornings and want to keep going through the night, then you are an ’owl’.
These terms have been used to describe the real phenomenon of diurnal preference – the times when you prefer to sleep and when you do your best work. Diurnal preference is determined partly by our clock genes. Exciting research in recent years has shown that small changes in these genes have been linked to the fast clocks (shorter than 24h) of larks or slower clocks (longer than 24h) of owls. But it is not just our genes that regulate our diurnal preference. Sleep timing changes markedly as we age.
By the time of puberty, bedtimes and wake times drift to later and later hours. This tendency to get up later continues until about the age of 19.5 years in women and 21 years in men. At this point, there is a reversal and a drift towards earlier sleep and wake times. By the age of 55–60 we are getting up as early as we did when we were 10.
These and allied results demonstrate that young adults really do have a problem getting up in the morning. Teenagers show both delayed sleep and high levels of sleep deprivation because they are going to bed late but still having to get up early in the morning to go to school. These real biological effects have been largely ignored in terms of the time structure imposed upon teenagers at school.
Of the few studies undertaken, later starting times for schools have been shown to improve alertness and the mental abilities of students during their morning lessons. Ironically, whilst young adults tend to improve their performance across the day, their older teachers show a decline in performance over the same period! The mechanisms for this change in diurnal preference remain poorly understood but are thought to relate to the marked changes in our steroid hormones (e.g. testosterone, oestrogen, progesterone) and their rapid rise during puberty and subsequent slower decline.
Light clocks and alertness
The eye establishes a connection to the outer world not only for our sense of sight but also for our sense of time and for many temporal processes in our body.
A clock is not a clock unless it can be set to local time – and the molecular clocks within the SCN are normally adjusted (entrained) by daily exposure to light around dawn and dusk detected by the eyes. Failure to expose the clock to a stable light/dark cycle results in drifting or ‘free-running ’ circadian rhythms or disrupted cycles.
Detachment from solar day is common in industrialised societies and the special case of shift workers will be discussed below; however, isolation from robust dawn and dusk signals occurs in many different instances. For example, paediatric and adult intensive care units frequently utilise low and constant light. In such an environment circadian rhythms would be expected to drift and become desynchronised. The result, as discussed below in the sub-section ’Disrupting the clock’, will be a weakened health status of the patient. Light does more than regulate the timing of circadian rhythms – it also has a direct effect on alertness and performance.
Brain imaging following light exposure shows increased activity in many of the brain areas involved in alertness, cognition and memory (thalamus, hippocampus, brainstem) and mood (amygdala). Furthermore, increased light has been shown to improve concentration, the ability to perform cognitive tasks and to reduce sleepiness. As a result, inappropriate light exposure in a building will not only disrupt sleep and circadian timing but also levels of alertness and performance.
Special photoreceptors in the ganglion cells of the optic nerve synchronise our inner clock with the cycles of light and dark in our environment – and thus with local time.
Our understanding of how light regulates circadian rhythms and alertness has been advanced dramatically over the past few years with the discovery of an entirely new photoreceptor system in the eye. This novel photoreceptor is not located in the part of the eye containing the rods (night vision) and cones (day vision) that are used to generate an image of the world, but in the ganglion cells that form the optic nerve. Most ganglion cells form a functional connection between the eye and the brain, but a small number of specialised ganglion cells (1–3%) are directly light-sensitive and project to those parts of the brain involved in the regulation of circadian rhythms, sleep, alertness, memory and mood.
These photosensitive retinal ganglion cells (pRGCs) contain a light-sensitive pigment called Opn4, which is most sensitive in the blue part of the spectrum with a peak sensitivity at 480 nm – very similar to the ‘blue’ of a clear blue sky. This light-detection system has evolved to be anatomically and functionally independent of the visual system, and probably evolved before vision as the main way to detect light for entraining daily rhythms. Remarkably, the pRGCs can still detect light to shift the circadian clock or affect alertness even in animals or people where the rods and cones used for vision are completely destroyed and who are otherwise totally visually blind. This raises important implications for ophthalmologists who are largely unaware of this new photoreceptor system and its impact on human physiology.
In view of the colour sensitivity of Opn4, we would predict that blue light should be the most effective wavelength (colour) for shifting circadian rhythms and alerting the arousal systems. In all studies undertaken to-date, this has been shown to be the case. Blue light exposure at night is most effective at shifting the timing of the circadian clock, reducing sleepiness, improving reaction times and activating areas of the brain mediating alertness and sleep.
In addition to its spectrum, light timing, duration, pattern and history all interact to influence circadian rhythms and alertness. Light timing is particularly important. Light can either advance (go to bed earlier) or delay (go to bed later) the circadian system depending on the timing of exposure. Under conditions of solar light exposure, light around dusk causes a delay of the clock, whereas light exposure around dawn will advance the clock. This delaying and advancing effect of light keeps the SCN locked onto to the solar day.
Such differential effects of light become vitally important when trying to understand the impact of jet lag, shift work (see below), or building design on sleep/wake timing. The pRGCs are not as sensitive to light as the rods and cones, so that short light exposure that is easily detected by the visual system is not recognised by the pRGCs. However, dim light can have an effect if it is delivered over long periods of time. Thus relatively dim indoor room light from bedside lamps and computer screens (less than 100 lux) can have measurable effects on the clock and arousal systems over several hours, and may exacerbate sleep disorders.
Collectively, these effects of light – spectral composition, time of exposure and brightness – have widespread clinical and occupational applications in not only treating sleep disorders and fatigue but in the architecture of hospitals, schools, offices, retail space and domestic buildings.