6. Sleep

Almost all life shows a 24-hour pattern of activity and rest, as we live on a planet that revolves once every 24 hours, causing profound changes in light, temperature, and food availability. Species are adapted to a particular temporal niche just as they are to a physical niche. Activity at the wrong time often means death. We spend approximately 36 per cent of our lives asleep, which suggests this aspect of our 24-hour behaviour provides us with something of huge value. ‘Sleep: The most obvious 24-hour rhythm’ considers two questions: Why has almost all life evolved a 24-hour circadian pattern of activity and rest? And what are the important processes that occur in the body during sleep?

3. When timing goes wrong

While time of day, interacting with an individual’s chronotype, can have an important impact upon performance and health, severe disruption of the circadian system adds another level of complexity and severity. ‘When timing goes wrong’ considers the effects of flying across multiple time zones, resulting in jet lag, and shift work on human health. Sleep and circadian rhythm disruption is almost always associated with poor health. Four circadian rhythm sleep disorders have been identified: advanced sleep phase disorder, delayed sleep phase disorder, freerunning, and irregular sleep timing. Sleep and circadian rhythm disruption in mental illness and neurodegenerative disease is also discussed.

7. Circadian rhythms and metabolism

Life is a continual battle to stay in a non-equilibrium energy state. Living things must obtain energy from the environment and effectively use it to drive metabolic reactions. ‘Circadian rhythms and metabolism’ shows that the circadian timing of energy metabolism, of which blood glucose levels are a major part, is a multi-layered system involving control by the suprachiasmatic nucleus, as well as by peripheral clocks in organs such as the liver, pancreas, muscle, and white adipose tissue. Several hormones also play an important role. The complex interactions between the suprachiasmatic nucleus, hypothalamus, hormones, and other signalling molecules involved in the circadian regulation of metabolism is mirrored at the subcellular level.

4. Shedding light on the clock

Most circadian clocks make use of a sun-based mechanism as the primary entraining signal to lock the internal day to the astronomical day. For nearly four billion years, dawn and dusk has been the main zeitgeber that allows entrainment. Circadian clocks are not exactly 24 hours. So to prevent daily patterns of activity and rest from freerunning over time, light can reset the clock. ‘Shedding light on the clock’ explains that the main circadian clock has been located in the suprachiasmatic nucleus in the hypothalamus. This also regulates the activity of the autonomic nervous system, but there are clocks in virtually every cell in the human body. Other zeitgebers include food, physical exercise, and temperature.

1. Circadian rhythms

‘Circadian rhythms: A 24-hour phenomenon’ explains the internally generated ‘clock’ that almost all living beings on Earth possess, which allows them to optimize a vast array of physiological processes and behaviour in advance of the varied demands of the daily solar cycle. Circadian rhythms are encoded in organisms’ genes, endogenously generated, and show near 24-hour rhythms in biological processes. They persist under constant conditions for several cycles, are entrained to the astronomical day via synchronizing zeitgebers, and show temperature compensation such that the period of the oscillation does not alter appreciably with changes in environmental temperature.

5. The tick-tock of the molecular clock

Circadian clocks in animals and plants arise from multiple and interconnected transcription–translation feedback loops that ensure the proper oscillation of thousands of genes in a tissue-specific manner. ‘The tick-tock of the molecular clock’ explains the transcription–translation feedback loop by describing the studies of circadian rhythms in Drosophila melanogaster, the fruit fly. The generation of a robust circadian rhythm entrained by the environment is achieved via multiple elements including the rate of transcription, translation, protein complex assembly, phosphorylation, other post-translation modification events, movement into the nucleus, transcriptional inhibition, and protein degradation. Similar mechanisms have been found in mammals, and insight is provided regarding research into how the mammalian molecular clock is entrained by light.

8. Seasons of life

With each season there are changes in day length, wind speed and direction, temperature, and rainfall, which dictate the optimal times for plants and animals to initiate growth, development, and reproduction. Plants are sessile and cannot escape or hide from seasonal change. Getting the timing right so that germination, bud burst, flowering, dormancy, and other critical processes are aligned with seasonal changes is essential for survival. Animals have more choices. They can migrate, hibernate, or, like plants, adapt their physiology and behaviour to cope with the changes. ‘Seasons of life’ explains that the most stable indicator of the time of year is photoperiod (day length) and considers the different light-detecting mechanisms in plants and animals.

2. Time of day matters

Numerous studies have shown that a broad range of activities—both physical and cognitive—vary across the 24-hour day. The blood pressure and heart rate of a human shows striking day–night variations. Simply, an organism is set up to function differently at different times of the day. ‘Time of day matters’ outlines the different human chronotypes—the different preferred waking and sleeping times of humans—and explains concepts such as ‘social jet lag’. There are optimal times of day for physical and cognitive activities, and chronotherapy has also shown that the administration of medication at different times of day can have an important impact on efficacy.

9. Evolution and another look at the clock

‘Evolution and another look at the clock’ concludes that the circadian system controls every key aspect of biochemistry, physiology, and behaviour. All plants, fungi, protista, algae, invertebrates, vertebrates, cyanobacteria, and at least one archaeon display circadian rhythms. These rhythms have many molecular components conserved between diverse species and are thought to be generated by molecular transcription–translation feedback loops in which a group of core clock genes regulate each other to ensure that their mRNA levels oscillate with a period of approximately 24 hours. Although the empirical evidence has been difficult to demonstrate, there is a consensus that the internal clock has been of vital importance in the evolutionary history of living things.