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EVER WALKED INTO the kitchen at the exact moment the oven beeps to say your dinner is ready? Most of us can estimate time passing with amazing precision thanks to a complex network of neural mechanisms. Over the past five years, our understanding of these has expanded rapidly, making it clear there is no single timekeeper in our brains. Nor is there a particular neural pathway that relates to time, as there is for other senses, such as smell or sight.

Some timekeeping happens unconsciously 鈥� our bodies have circadian rhythms that help biological processes occur at the right time of day (see 鈥淐an we live without time?鈥�). But mechanisms in our brain also contribute to our ability to sense the passing of time.

Let鈥檚 start with 鈥渢ime cells鈥�. These reside in the hippocampus 鈥� a brain region involved in memory 鈥� and were . Recently, time cells were also identified in humans by and his colleagues. Lega says that to understand what time cells do, first we need to consider 鈥減lace cells鈥� 鈥� brain cells that mark where in a spatial environment an animal is. If the animal walks along a linear tunnel, place cells would fire in a linear manner. Time cells do something similar, but for the passage of time.

When you are at an event 鈥� a movie or a dinner party, for instance 鈥� your brain will recognise this as a specific, or notable, chunk of time that neuroscientists call an 鈥渆pisode鈥�. In many cases, our brains know how long each episode is likely to be. 鈥淭he brain has a rough template of most situations,鈥� says Lega. Armed with this knowledge, each time cell will fire in different windows during each episode. One might fire after 10 seconds, another at 2 minutes, and so on.

Time cells might explain why certain experiences feel like they happen faster than others (see 鈥淲hat affects our perception of time?鈥�). Lega says the subjective experience of time may depend on how many episodes you are creating, and the detail you assign to each one. In an intense experience, your brain might create multiple episodes with many time cells associated with many events, making time appear to slow down.

To track the passing of time, we may also need 鈥渞amping cells鈥�. Whereas time cells wait until it is their turn to fire, a ramping cell will fire intensely at the beginning of an episode, then gradually slow down. 鈥淩amping cells will coordinate with time cells,鈥� says Lega. 鈥淭hey are two complementary ways of mapping temporal information.鈥�

So how do we stitch together the passage of time? The brain combines information from place cells about where an event is happening and sensory information about what is going on 鈥� what a person鈥檚 perfume was, what they were talking about. Then, add in the time and ramping cell activity and the brain can put all the information into the appropriate time frame. This data is packaged up into episodic memories and stored, allowing us to perceive the temporal order of our lives.

This is the leading theory. But there may be other mechanisms involved, too, says Lega. Theta rhythms 鈥� waves of brain activity that oscillate at around 4 to 8 hertz 鈥� may also timestamp activities, depending on where within the wave they occur. 鈥淪ome researchers believe the brain can tell time without time and ramping cells entirely鈥� he says.