The clock is tricking: jet lag and the natural rhythm of teenagers

This summer, if you are lucky enough to be going on travels that take you across several time zones, you are likely to experience an uncomfortable dissonance between your internal biological clock and external astrophysical time. In other words, you will get jet lag.

The most familiar symptom of jet lag is sleepiness at the wrong time of day. However, this is only the most outwardly obvious characteristic of a complex internal system that regulates a diverse range of bodily functions over an approximate 24-hour cycle.

A growing number of physiological functions, including body temperature, digestive function, physical strength, blood pressure, immune function and mental alertness are known to oscillate in a circadian rhythm.

Jet lag happens because these cycles run somewhat autonomously. If you abruptly change time zones, as when flying, your body's internal clock may still be set to GMT even though you have moved five time zones east. There is no known shortcut for resetting these rhythms. With jet lag, not only do you get tired but you can also experience insomnia, bowel irregularity, swollen limbs, disorientation, headaches and irritability. It takes days for your body to adjust to the new time zone and for you to regain a rhythm that fits with the day and night of your surroundings.

The cells of your body have their own internal clocks, where the mechanics are not interlocking brass cogs but genes and their proteins. These keep time through a system of feedback loops. In these loops, clock genes get turned on, and so start to produce clock proteins. When the proteins reach high enough concentrations, they switch off the clock genes, which remain off until the protein levels have dropped sufficiently. And so on.

The master co-ordinator of all this is in an area of the hypothalamus of the brain containing the suprachiasmatic nuclei. These nuclei are necessary to keep all the little clocks synchronised so that the cycle of feedback loops begins and ends at the same time in all the cells of your body.

These cycles take roughly 24 hours, but not exactly. If you were living in a darkened room, the cycle would gradually go out of sync with daylight hours. After spending sufficient time in darkness before re-emerging into the day (or night), your patterns of wakefulness and sleep would mismatch the environment, much like in the case of jet lag. In order to keep proper circadian rhythm, the clock needs to be calibrated with light.

We are used to the idea of light as a visual stimulus, but it is more than that. The rod and cone cells of the retina contain opsins – proteins that respond to light and send signals to the brain – which are interpreted as images. The observation that animals lacking rod and cone cells still regulate their circadian rhythms with light led to the discovery of a whole new type of opsin called melanopsins, which are sensitive to blue light but are not involved in vision. Instead they signal to the suprachiasmatic nuclei and calibrate the body clock with daylight hours. This readjustment is not instantaneous, which is why it takes a few days to get over jet lag.

Teen time

Remarkable progress has been made in understanding circadian rhythms in recent years. One interesting discovery is that, among the other disruptions caused by adolescence, the circadian rhythm is altered to run later. Teenagers prefer to go to bed late and get up late, not out of petulance but because that is how their body clocks run.

It has been estimated that, while adults are usually fully alert by 10am, most teenagers take until noon, and about 10-20 per cent of teenagers are not fully alert until 2pm. Some people have argued that a school system requiring all students to start lessons at 9am is forcing teenagers into an unnatural rhythm.

The rhythm of life that cycles with day and night is not unique to animals, and has been found in nearly all forms of life, including bacteria. The symptoms of jet lag are the consequence of trying to regain this rhythm after disruption. In the roughly three billion years since life originated on this planet, the environment has drastically changed many times over. That biological life has synchronised with the astrophysical cycle is not surprising when you consider that one thing has remained constant throughout all this time: every day begins with dawn and ends with dusk.

Aoife McLysaght is a professor in genetics in TCD, where she leads a research group focusing on identifying and interpreting the evolutionary patterns in animal genomes