THE BRAIN FROM TOP TO BOTTOM

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The Sleep/ Dream/ Wake Cycle

Our Biological Clocks

Brain Imaging

Nocturnal animals too have circadian rhythms, even though these animals are active at night and rest in the daytime. Cockroaches, for example, are very good models for research on circadian rhythms, because their activity can be readily recorded and their nervous systems are relatively simple and a convenient size.

When mice are placed in temporal isolation, their autonomous circadian rhythm has a period of about 23.5 hours. In hamsters , it is close to 24 hours, and in humans, it ranges from 24.2 to 25.5, depending on which studies you read. This period is thus genetically determined by the biological clock in each species. Interestingly, a mutation in even just one of the many genes involved in this clock can modify its period.

Jet lag and working night shifts represent two very particular sets of conditions in which people’s circadian rhythms are suddenly thrown out of phase.

The changes in the seasons have a similar effect, but over a much longer period. The reason that the changing seasons upset people’s circadian rhythms is that people tend to get out of bed at just about the same time every day, because of their family and work obligations, whereas the time that they go to sleep varies more. As the seasons change, the times of sunrise and sunset change considerably, so when someone keeps getting up at the same hour, their system gets the impression that time is somehow shifting. Seasonal affective disorder, experienced in winter by some people in Northern countries where the days are very short, may well be related to this shift in the body’s circadian clock.

CHRONOBIOLOGY

Our natural environment is filled with cyclical patterns and rhythms whose ultimate origins lie in the particular configuration of our solar system. For example, our seasonal fluctuations in temperatures and precipitation originate in the annual revolution of the Earth around the Sun and the tilt of the Earth’s axis of rotation.

Other phenomena, such as tides, are driven by the monthly revolution of the Moon around the Earth.

And, of course, the alternation of day and night is due to the Earth’s rotation on its axis, with one complete rotation taking approximately 24 hours.

The human brain, because it has always evolved in this cyclical environment, has adapted to it in many ways. The most obvious is the sleep-wake cycle, which follows the alternation of day and night. But the mammalian brain can also orchestrate cyclical behaviours with periods much longer than a day, such as hibernation, or much shorter, such as the respiratory cycle. The cerebral cortex itself displays cycles whose periods are even shorter, with frequencies of tens of cycles per second.

Thus, cycles are omnipresent in the mammalian nervous system. Their frequencies can range from 100 cycles per second (100 Hertz) for cortical EEGs to just once per year (or 0.00000003 Hz) for many seasonal behaviours, such as the mating of deer in autumn, or hibernation in bears.

The study of these cycles (also known as rhythms) in biological organisms is called chronobiology. By focusing on the time dimension of human physiological and psychological activities, chronobiology has provided a better understanding of many phenomena, including why certain medications are more effective when taken at particular times of day.

A cycle or rhythm is present when a phenomenon repeatedly goes through a peak and a trough over a specific period of time. As just noted, the duration of this period can vary greatly. That is why biological rhythms are generally classified into three major categories according to their period—the time interval separating one peak or trough from the next in a repeating cycle.

Ultradian rhythms have a period of less than 24 hours. One example is the alternation of deep sleep and REM sleep in human beings, which occurs about once every 90 minutes when someone is asleep.

Circadian rhythms have a period of about 24 hours (the word “circadian” comes from the Latin circa , meaning “around” and dies, meaning “a day”). There are many examples of circadian rhythms, such as the sleep-wake cycle, the body-temperature cycle, and the cycles in which a number of hormones are secreted.

Infradian rhythms have a period of more than 24 hours. The menstrual cycle in women and the hibernation cycle in bears are two good examples.

In 1729, the French physicist Jacques d’Ortous de Mairan observed that the mimosa shrub “knew” when it was daytime and when it was nighttime, because it opened and closed its leaves, respectively, at those times, even when it was enclosed in a box that let in no light. This plant was therefore responding not to the sun but to an internal biological clock. Today we know that most living organisms have their own biological clocks.