How do you know when it’s time to wake up or go to sleep? More potent than any alarm is your circadian rhythms. In this animation, we take a look at how these rhythms work and what controls them, inspired by the TeenSleep project being carried out at the University to look at how later start times at school might affect achievement. How does our body know when it’s time to sleep? Humans detect light through the eye. Light enters the eye and is focused onto the retina at the back of the eye. The retina contains photoreceptive cells that detect light and send this information to the brain, via the optic nerve. The most obvious outcome of this process is the ability to form images; to see. Image-forming vision depends upon rod and cone photoreceptors, which are critical for low light vision and bright light color vision, respectively. However, it was discovered in 1999 that the circadian system of animals lacking rods and cones could still respond to light. This led to the discovery of a new class of photoreceptive cells: the photosensitive retinal ganglion cells (pRGCs), which express the light sensitive pigment melanopsin. These pigments undergo a chemical reaction when they absorb light, which causes the ganglion nerve cells to fire signals to the brain. These signals from the melanopsin pRGCs feed directly into the Suprachiasmatic Nuclei (SCN). These are cells in the hypothalamus, near the base of the brain, which contains the master circadian clock (or pacemaker). The SCN orchestrates our circadian processes, to make sure that systems throughout our bodies are working together, in time. This system is also involved in controlling our sleep-wake cycle. As the level of light gradually decreases at the start of the night, we produce increasing levels of a hormone called melatonin. Melatonin plays a vital role in our day-night cycles, acting as an internal signal of the night time. Why is being exposed to light at the wrong time so bad? If we are exposed to light at the wrong time, say because we work night shifts, this confuses our system. Light suppresses the production of melatonin, and promotes wakefulness. We delay our sleep and other circadian and sleep-dependent processes. We can work nightshifts for years, and our circadian system will not adapt to our new sleep-wake cycle – primarily because we are exposed to natural light during the day, which is far brighter than artificial light sources. This leads to a whole host of problems. Working through the night means we are working when our bodies are craving sleep. Is it different for teenagers? The reason we are so interested in sleep during adolescence is that our circadian rhythms change during this period. From the age of 10 until around 21, our circadian rhythms delay. This means that as we go through adolescence and into early adulthood, we are naturally more inclined to go to bed later and also to get up later. This is a biological process and will happen to teenagers regardless of their environment.