In 1962, a cave explorer named Michel Siffre embarked on a groundbreaking journey to understand how our bodies perceive time. He isolated himself in a dark cave for months, devoid of any light or clocks, and monitored his vital signs, sleep, and eating patterns using electrodes. When he finally emerged, the results were astonishing. Despite the absence of external time cues, Siffre’s body maintained a consistent sleep-wake cycle, a phenomenon known as the circadian rhythm, which means “about a day” in Latin. This discovery laid the foundation for the field of chronobiology, which explores how these rhythms influence everything from hormone secretion to drug effects.
Our ability to sense time is crucial for daily activities, from waking up to catching a ball. This capability is thanks to a complex network of timekeepers in our brains. These include mechanisms akin to a stopwatch for seconds, a clock for hours, and a calendar for seasons, each located in different brain regions. Siffre, during his cave experiment, relied on the most basic timekeeper, the suprachiasmatic nucleus (SCN) in the hypothalamus.
Research on fruit flies and mice has provided insights into how our biological clock functions. Proteins known as CLK (clock) accumulate in the SCN throughout the day. These proteins activate genes that keep us awake and produce another protein called PER. As PER builds up, it eventually deactivates the gene responsible for CLK, leading to sleep. As CLK levels drop, PER levels also decrease, allowing CLK to rise again, thus restarting the cycle. This seesaw effect between CLK and PER is a key driver of our day-night cycle.
For greater accuracy, our SCNs also depend on external cues known as zeitgebers, which means “givers of time” in German. These include light, food, noise, and temperature. While Siffre lacked these cues in his cave, in everyday life, they help fine-tune our daily behaviors. For example, morning light entering our eyes signals the SCN through the optic nerve, prompting the hypothalamus to stop producing melatonin, the sleep hormone, and increase vasopressin and noradrenaline, which regulate sleep cycles.
Around 10 AM, our body’s rising temperature enhances energy and alertness, and later in the afternoon, it boosts muscle activity and coordination. However, exposure to bright screens at night can disrupt these signals, making it harder to fall asleep. For precise timekeeping, our brain’s internal stopwatch comes into play. One theory suggests that communication between neurons takes a consistent amount of time, allowing the cortex to judge time intervals accurately, shaping our perception of time.
During his cave experiment, Siffre made an intriguing discovery. He challenged himself daily to count to 120 at a rate of one digit per second. Over time, this task, which should have taken two minutes, began taking him up to five minutes. The isolation and darkness of the cave distorted his perception of time, despite his brain’s efforts to maintain accuracy. This raises intriguing questions about what influences our sense of time and whether each of us experiences it differently. Only time will tell.
Keep a detailed journal for one week, noting your sleep patterns, energy levels, and mood at different times of the day. Reflect on how these patterns align with the concept of circadian rhythms discussed in the article. Share your findings with classmates to compare and contrast your experiences.
Organize a controlled environment where you and your peers spend a few hours without external time cues such as clocks or natural light. Record your perception of time and discuss how it compares to Michel Siffre’s experience. Consider what factors might influence your internal timekeeping.
Conduct a research project on how different zeitgebers like light, food, and temperature affect the circadian rhythm. Present your findings in a group presentation, highlighting how these external cues can be manipulated to improve sleep and productivity.
Participate in a debate on the impact of modern technology, such as smartphones and computers, on our biological clocks. Use evidence from the article and additional research to argue whether technology is a help or hindrance to maintaining healthy circadian rhythms.
Try the counting exercise that Siffre used in his cave experiment. Count to 120 at a rate of one digit per second, and note how long it actually takes you. Discuss with classmates how isolation or lack of external cues might alter your perception of time, as it did for Siffre.
In 1962, a cave explorer named Michel Siffre started a series of experiments where he isolated himself underground for months without light or clocks. He attached himself to electrodes that monitored his vital signs and tracked when he slept and ate. When Siffre finally emerged, the results of his pioneering experiments revealed that his body had maintained a regular sleeping-waking cycle. Despite having no external cues, he fell asleep, woke up, and ate at fixed intervals. This became known as a circadian rhythm, from the Latin for “about a day.” Scientists later found that these rhythms affect our hormone secretion, how our bodies process food, and even the effects of drugs on our bodies. The field of science studying these changes is called chronobiology.
Being able to sense time helps us do everything from waking and sleeping to knowing precisely when to catch a ball that’s coming towards us. We owe these abilities to an interconnected system of timekeepers in our brains. It contains the equivalent of a stopwatch telling us how many seconds have elapsed, a clock counting the hours of the day, and a calendar notifying us of the seasons. Each one is located in a different brain region. Siffre, isolated in his dark cave, relied on the most primitive clock in the suprachiasmatic nucleus (SCN) of the hypothalamus.
Here’s the basics of how we think it works based on studies with fruit flies and mice. Proteins known as CLK, or clock, accumulate in the SCN throughout the day. In addition to activating genes that keep us awake, they produce another protein called PER. When enough PER accumulates, it deactivates the gene that makes CLK, eventually leading to sleep. As CLK levels drop, PER concentrations also decrease, allowing CLK to rise again, starting the cycle over. There are other proteins involved, but our day and night cycle may be driven in part by this seesaw effect between CLK during the day and PER at night.
For more precision, our SCNs also rely on external cues like light, food, noise, and temperature. We call these zeitgebers, German for “givers of time.” Siffre lacked many of these cues underground, but in normal life, they fine-tune our daily behavior. For instance, as natural morning light filters into our eyes, it helps wake us up. Traveling through the optic nerve to the SCN, it communicates what’s happening in the outside world. The hypothalamus then halts the production of melatonin, a hormone that triggers sleep. At the same time, it increases the production of vasopressin and noradrenaline throughout the brain, which help control our sleep cycles.
At about 10 AM, the body’s rising temperature boosts our energy and alertness, and later in the afternoon, it also improves our muscle activity and coordination. Bright screens at night can confuse these signals, which is why watching TV before bed can make it harder to sleep. But sometimes we need to be even more precise when telling the time, which is where the brain’s internal stopwatch comes in. One theory for how this works involves the fact that communication between a given pair of neurons always takes roughly the same amount of time. So neurons in our cortex and other brain areas may communicate in scheduled, predictable loops that the cortex uses to judge with precision how much time has passed. That creates our perception of time.
In his cave, Siffre made a fascinating additional discovery about this. Every day, he challenged himself to count up to 120 at the rate of one digit per second. Over time, instead of taking two minutes, it began taking him as long as five. Life in the lonely, dark cave had warped Siffre’s perception of time despite his brain’s best efforts to keep him on track. This raises questions about what else influences our sense of time. And if time isn’t objective, what does that mean? Could each of us be experiencing it differently? Only time will tell.
Circadian – Relating to biological processes that occur on a roughly 24-hour cycle, even in the absence of external cues. – The circadian rhythm in humans is influenced by light exposure, which helps regulate sleep-wake cycles.
Rhythm – A regular, repeated pattern of movement or sound, often used to describe biological cycles. – The cardiac rhythm is crucial for maintaining effective blood circulation throughout the body.
Chronobiology – The study of periodic phenomena in living organisms and their adaptation to solar and lunar related rhythms. – Chronobiology explores how the timing of medication administration can affect its efficacy and side effects.
Suprachiasmatic – Referring to the suprachiasmatic nucleus, a region of the brain responsible for controlling circadian rhythms. – The suprachiasmatic nucleus receives direct input from the eyes and helps synchronize the body’s internal clock with the external environment.
Nucleus – A membrane-bound organelle found in eukaryotic cells that contains the genetic material. – The nucleus plays a critical role in regulating gene expression and maintaining the integrity of genetic information.
Proteins – Large, complex molecules made up of amino acids that perform a vast array of functions within organisms. – Proteins are essential for catalyzing metabolic reactions, replicating DNA, and transporting molecules.
Zeitgebers – External cues that synchronize an organism’s internal clock to the environment, such as light and temperature. – Light is the most powerful zeitgeber, helping to reset the circadian rhythms of organisms to align with the day-night cycle.
Melatonin – A hormone produced by the pineal gland that regulates sleep and wakefulness. – Melatonin levels typically rise in the evening, promoting sleepiness and helping to regulate the sleep-wake cycle.
Neurons – Specialized cells in the nervous system that transmit information through electrical and chemical signals. – Neurons communicate with each other via synapses, forming complex networks that underlie all nervous system functions.
Perception – The process by which organisms interpret and organize sensory information to represent and understand the environment. – Visual perception allows organisms to interpret light signals and form images of their surroundings.
