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Unlock This CourseHave you ever wondered why you feel sleepy at night and alert during the day? This isn't just habit - it's controlled by powerful biological clocks inside your body called circadian rhythms. These internal timekeepers run on roughly 24-hour cycles and control many of your body's functions, including when you sleep and wake up.
Key Definitions:
Your body has a master clock in your brain called the suprachiasmatic nucleus (SCN). This tiny cluster of cells acts like a conductor of an orchestra, coordinating all your body's rhythms. It receives information about light and darkness through your eyes and adjusts your internal clock accordingly.
Your circadian rhythms don't just randomly decide when you should sleep. They work with two main systems: your internal body clock and something called sleep pressure. Sleep pressure builds up the longer you stay awake, making you feel increasingly tired.
Light is the most powerful influence on your circadian rhythms. When light hits special cells in your eyes, they send signals to your brain's master clock. This is why bright light in the morning helps you wake up, whilst darkness in the evening makes you feel sleepy.
Bright light in the morning tells your brain it's time to be awake and alert. This stops the production of melatonin, a hormone that makes you sleepy.
When it gets dark, your brain starts producing melatonin, which makes you feel drowsy and prepares your body for sleep.
Bright artificial light, especially blue light from screens, can trick your brain into thinking it's still daytime, making it harder to fall asleep.
Your sleep-wake cycle is like a daily rhythm that your body follows. For most people, this cycle makes them feel most alert during the day and sleepy at night. But it's not just about being awake or asleep - there are specific times when you naturally feel more or less alert.
6:00 AM: Body temperature starts to rise
9:00 AM: Peak alertness begins
2:00 PM: Natural dip in alertness (post-lunch dip)
6:00 PM: Body temperature peaks
9:00 PM: Melatonin production begins
10:30 PM: Optimal bedtime for most people
2:00 AM: Deepest sleep occurs
4:30 AM: Lowest body temperature
Not everyone has the same sleep-wake cycle. Some people are naturally "morning larks" who wake up early and feel most alert in the morning. Others are "night owls" who prefer to stay up late and sleep in. These differences are partly genetic and partly influenced by age.
About 25% of people are morning types. They naturally wake up early, feel most alert in the morning and prefer to go to bed early. Their body temperature rises earlier in the day.
About 25% of people are evening types. They naturally stay up late, feel most alert in the evening and struggle with early mornings. Their body temperature peaks later in the day.
When your circadian rhythms are disrupted, it can cause serious problems with your sleep, mood and overall health. This can happen for many reasons, from shift work to jet lag to simply using bright screens late at night.
Modern life often conflicts with our natural circadian rhythms. Artificial lighting, irregular work schedules and international travel can all throw off our internal clocks, leading to various sleep disorders and health problems.
Sarah works as a nurse on rotating night shifts. She often struggles to sleep during the day and feels exhausted at work. Her circadian rhythms are constantly being disrupted because she's trying to sleep when her body expects to be awake. This is called Shift Work Sleep Disorder, affecting about 10% of night shift workers. Symptoms include insomnia, excessive sleepiness and difficulty concentrating. Treatment involves light therapy, melatonin supplements and creating a dark, quiet sleep environment during the day.
Jet lag happens when you travel across time zones faster than your body can adjust its internal clock. Your circadian rhythms are still set to your home time zone, but you're trying to function in a new one. This creates a mismatch that can last several days.
Flying east is generally harder on your body because you're "losing" time. Your body finds it more difficult to advance its clock than to delay it. Recovery typically takes about one day per time zone crossed.
Flying west is usually easier because you're "gaining" time. Your body finds it easier to delay sleep than to advance it. Recovery is typically faster than eastward travel.
Alongside your circadian rhythms, your body has another system that controls sleep called sleep pressure or sleep drive. This builds up the longer you stay awake, like a battery running down. The chemical adenosine accumulates in your brain throughout the day, making you feel increasingly tired.
Think of sleep pressure like thirst - the longer you go without water, the thirstier you become. Similarly, the longer you stay awake, the stronger your drive to sleep becomes. During sleep, your brain clears away the adenosine, "recharging" your alertness for the next day.
Tom is a 16-year-old student who can't fall asleep before 2 AM, no matter how tired he feels. He naturally wakes up around 11 AM but has to get up at 7 AM for school, leaving him constantly sleep-deprived. This is Delayed Sleep Phase Disorder, common in teenagers due to natural changes in their circadian rhythms during puberty. Treatment involves gradually shifting bedtime earlier, morning light therapy and sometimes melatonin supplements taken in the early evening.
Understanding circadian rhythms can help you improve your sleep and daily performance. Simple changes to your routine can help keep your internal clock running smoothly and improve your overall wellbeing.
You can support your natural rhythms by maintaining consistent sleep and wake times, getting bright light exposure in the morning, avoiding screens before bedtime and creating a dark, cool sleeping environment.
Get bright light within an hour of waking up. Spend time outdoors or near a bright window. This helps reset your internal clock each day.
Keep consistent bedtime and wake times, even on weekends. Create a relaxing bedtime routine and avoid caffeine late in the day.
Make your bedroom dark, quiet and cool. Use blackout curtains or an eye mask to block out light that might disrupt your sleep.
Your circadian rhythms change throughout your life. Babies don't have fully developed rhythms, which is why they wake up frequently during the night. Teenagers naturally shift to later bedtimes and wake times, whilst older adults often become more morning-oriented and may experience fragmented sleep.
Research by Dr Mary Carskadon showed that teenagers' circadian rhythms naturally shift later during puberty. This biological change means teens don't start producing melatonin until around 11 PM, making it difficult for them to fall asleep early. This research has led some schools to start later in the morning, resulting in improved academic performance and reduced absenteeism.