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According to their model, the striatum monitors activity in other areas of the brain, including the frontal cortex. As neurons in these regions perform their usual functions – coordinating memory and movement, for example – they emit repeating pulses of electrical activity. As Meck explains, "All of these neurons are oscillating on their own schedules; none of them are synchronised."

But when a stimulus grabs our attention – when we place a pot on the stove, for instance – it prompts all the neurons in the cortex to fire simultaneously, causing a spike in electrical output. Afterwards the neurons resume their disorderly pulsing, but because of their synchronised start, they now create a distinct, reproducible pattern of oscillation.

"This pattern is like a symphony played by an orchestra," says Buhusi. "And each structure in the brain is a different instrument." Then, at the instant the stimulus ends – when the pot on the stove starts to boil – the striatum records the pattern of cortical oscillations. As Buhusi explains, "It's like noting where the orchestra is up to in the melody."

That point in the melody then acts as a reference for other encounters with the same stimulus. So when the striatum 'detects' the refrain after which a future pot should have boiled, it sends an electrical pulse to another brain region called the thalamus. The thalamus then communicates with the cortex so that higher cognitive functions such as memory and decision-making kick in, and the pot gets taken off the stove.

So while the circadian timepiece is like a clock that works without external cues, the interval-timing system functions more like a stopwatch. In other words, our conscious perception of time relies on constantly checking the duration of familiar external events. This explains Siffre's feeling of disorientation: in the darkness and isolation of his cave, his interval timing system lost its usual markers.

According to Meck, the neurotransmitter dopamine (which is central to our reward system) is also key to our interval-timing system. In his model, a burst of dopamine at the onset of a stimulus starts our internal stopwatch, and a second burst stops it at the end. He thinks dopamine also affects the frequency of neural oscillations (the orchestra's tempo), effectively speeding up or slowing down our inner clock.

This theory is backed by observations that diseases and drugs that affect dopamine levels distort our perception of time. Patients with untreated Parkinson's disease, for example, have low dopamine levels, and their slow internal clock makes them underestimate durations of time. Patients with schizophrenia, by contrast, have heightened dopamine activity, "and their clocks run so fast it feels like the whole world is crazy," says Meck.

Drugs that regulate dopamine activity can bring the internal clocks of Parkinson's and schizophrenic patients back to relatively normal speeds. On the other hand, recreational drugs that affect dopamine levels can wreak havoc with our perception of time.

Stimulants such as nicotine, caffeine and cocaine speed up the 'ticks' of the internal clock, making users feel as though time is passing more quickly. This leads them to overestimate how much time has passed, so five minutes might feel like fifteen. On the other hand, sedatives like marijuana and Valium slow down the internal 'ticking', leading to the opposite effect.

Extremes of temperature can also warp our experience of time. In the 1930s, American physiologist Hudson Hoagland noticed that when his wife was sick she seemed to overestimate periods of time. So, being a consummate professional, he conducted an experiment: throughout her illness Hoagland asked his wife to count to 60 while he took her temperature; the hotter she was the faster she would count.

Hoagland suspected the heat made his wife's internal clock speed up, so she felt as if more time had passed than actually had. Over the next few decades his observations inspired a series of bizarre experiments in which subjects wore specially-designed heating helmets or sat in sweat rooms kept at 63 °C, until some were on the verge of collapse.

"You'd never be allowed to do these experiments today," says John Wearden, professor of psychology at Keele University in Staffordshire, England. But you can draw on the results, and a few years ago Wearden analysed the data. His conclusion: while the mechanism remains unknown, raising the brain's temperature can alter a person's sense of time by more than 20 per cent.

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