When we sleep, our body alternates between non-REM (NREM) and rapid eye movement (REM) sleep. This alternation occurs in cycles throughout the night. On average, one cycle lasts about 90 minutes, and there are usually 4–6 cycles during a night’s sleep.
Waking up from NREM sleep is often more difficult and sluggish compared to waking up from REM sleep. Since REM sleep typically occurs at the end of a cycle, aligning your wake-up time with these cycles can help you feel more refreshed.
The calculator estimates suitable bedtimes based on sleep cycles while also taking into account the time it usually takes you to fall asleep. However, sleep cycle length can vary greatly between individuals, so scheduling sleep strictly around 90-minute cycles may not work for everyone. For this reason, you can also enter the amount of sleep you personally feel you need in order to wake up refreshed in the morning.
The light we perceive through our eyes plays a major role in our body's circadian rhythm. As the lights dim, our body senses that evening is coming and, for example, melatonin secretion increases, which helps us fall asleep. Bright lights in the environment and, for example, screens from electronic devices can reduce melatonin secretion and thus make it more difficult to fall asleep.
Try to stop looking at screens (phone, TV, computer, etc.) at least 1 hour before bedtime.
Our body temperature normally drops about 2 hours before we fall asleep. This acts as a signal to our body to go to sleep and makes it easier to fall asleep. A warm shower, bath or sauna lowers body temperature, which is why it can also help you fall asleep and improve the quality of your sleep.
If you have noticed that you wake up frequently at night to go to the bathroom, stopping or reducing your fluid intake 2 hours before bedtime may reduce awakenings.
It is a good idea to dim the surrounding lighting 3 hours before bedtime.
Drinking alcohol close to bedtime impairs sleep quality by delaying the onset of rapid eye movement (REM) sleep and shortening the time spent in REM sleep, and prolonging the time it takes to fall asleep and enter deep sleep. Nicotine consumption before bedtime also impairs sleep quality and increases nighttime awakenings. If you are going to drink alcohol and/or nicotine products, stop drinking them at least 4 hours before bedtime.
Avoid napping within 7 hours of bedtime.
Caffeine consumption close to bedtime increases the time it takes to fall asleep, reduces sleep quality, and increases nighttime awakenings. The last cup of coffee (about 107 mg of caffeine) should be consumed no later than 8.8 hours before bedtime to avoid negative effects on your sleep. The last large dose of caffeine (about 217.5 mg of caffeine) should be consumed no later than 13.2 hours before bedtime to avoid negative effects on your sleep.
Sleep is essential for both physical and mental health, and every human being needs it. Research shows that sleep helps the nervous system to mature, supports cognitive functions, removes toxins that build up in the brain while awake, and plays a role in learning and memory.
The effects of sleep are not limited to the brain. Sleep influences almost every system in the body, including the respiratory system, the circulatory system, the immune system, and metabolism.
During sleep, the body alternates between stages of non-REM (NREM) sleep and REM sleep. Together, these form sleep cycles that last about 90–110 minutes. A normal night includes 4 to 6 cycles.
Stage 1 – Light sleep (NREM 1)
Sleep begins with stage 1, which lasts about 1–5 minutes. It is very light sleep. Muscles relax, heart rate slows down, breathing becomes slower, and eye movements decrease.
Stage 2 – Deeper sleep (NREM 2)
Stage 2 is longer. In the first cycle it lasts about 25 minutes, and in later cycles it becomes longer. About 45% of total sleep time is spent in this stage. Heart rate and body temperature drop further, muscles relax more, and eye movements stop.
Stage 3 – Deep sleep (NREM 3)
Stage 3 is the deepest sleep. In this stage, the body repairs and grows tissues, strengthens the immune system, and builds bones and muscles. Heart rate and breathing are at their lowest levels, and muscles are fully relaxed. Waking up from this stage is difficult. If you wake up during stage 3, it is common to feel tired and less alert for 30–60 minutes.
REM sleep
REM sleep is the final stage of the sleep cycle. In this stage, brain activity looks similar to being awake, but the body’s muscles remain inactive. Only the eyes move rapidly, which is why it is called Rapid Eye Movement sleep.
REM sleep is important for brain development. Which may explain the fact that newborns spend about 8 hours per day in REM sleep, while adults spend about 2 hours. Premature babies and young animals also have more REM sleep.
In adults, the first REM stage at night lasts about 10 minutes, but it becomes longer in later cycles. The last REM stage before waking up can last up to one hour. Many people naturally wake up from REM sleep in the morning.
To complete enough sleep cycles during the night, sleep must be long enough. The American Academy of Sleep Medicine and the Sleep Research Society recommend at least 7 hours of sleep per night for adults.
Sleeping less than 7 hours regularly has negative effects on health. It increases the risk of heart disease, stroke, obesity, diabetes, and high blood pressure. The risks are especially high for people who sleep closer to 6.5 hours or less per night.
The amount of sleep is important, but the quality of sleep is also very important. Good quality sleep supports health, energy, and daily performance.
The body follows a cycle of about 24 hours, called the circadian rhythm. This rhythm regulates the sleep–wake cycle and many biological functions, such as hormone release and body temperature. It also affects cognitive performance, physical performance, appetite, and eating.
Changes in the circadian rhythm can cause poor sleep and even chronic diseases. Although the circadian rhythm works like an internal “biological clock,” our choices can influence it. With regular routines, we can improve sleep quality and daily energy levels.
“Rhythm” means a repeating pattern. Regularity is important for circadian rhythm. For better sleep, go to bed at the same time every night and wake up at the same time every morning.
Many factors can affect circadian rhythm, such as age, gender, work schedules, exercise, diet, and medication. People who work shifts often experience large variations in sleep. There are ways to adjust the rhythm and support better sleep quality.
Light is the strongest factor that controls the circadian rhythm. Our eyes sense light, which helps regulate body functions. Morning and evening light are especially important. Exposure to light in the morning moves the rhythm earlier. Evening light exposure moves the rhythm later.
Limit bright light and screen use in the evening. In the morning, get natural sunlight or turn on bright lights early. Using the sleep calculator can help you find the best times to reduce evening light and limit screens.
The circadian rhythm also controls body temperature. In healthy adults, temperature changes about 1 °C (1.8 °F) during the day. Body temperature is lowest about 2 hours before waking, then rises and reaches its peak about 2 hours before bedtime. After the peak, it falls again during the evening and night.
You can help your body sleep better by slightly lowering or raising body temperature. For example, a warm bath in the evening can help. Although it sounds strange, a warm bath lowers body temperature in the end. Warm blood moves to the skin and loses heat, especially through the hands and feet. This helps the body relax and feel sleepy. The sleep calculator can suggest the best time for a warm bath.
Healthy lifestyle habits, including diet and physical activity, also improve sleep quality. People who are overweight often sleep less. Poor sleep also negatively affects eating habits and weight control.
Eat a balanced, healthy diet. Avoid saturated fats, and prefer unsaturated fats. Fatty fish can improve sleep, likely due to omega-3 and omega-6 fatty acids and vitamin D, which help produce serotonin. Serotonin is a brain chemical that regulates sleep.
Protein intake is important. Too little protein (<16% of daily calories) can reduce sleep quality and make it harder to fall asleep. Too much protein (>19% of daily calories) can make staying asleep harder. Some protein sources contain tryptophan, which the brain uses to make serotonin.
For carbohydrates, focus on enough fiber.
If you wake up at night to use the bathroom, stop drinking liquids about 2 hours before bed. This can reduce night waking and improve sleep.
Exercise and daily activity have positive effects on sleep. General recommendations are 150 minutes of moderate exercise or 75 minutes of vigorous exercise per week, plus strength training twice a week.
Evening high-intensity exercise (≥77% of maximum heart rate) less than 4 hours before bedtime may reduce REM sleep and lower sleep quality. Low-intensity evening exercise (≤63% of max heart rate) can shorten the time it takes to fall asleep. Moderate-intensity evening exercise (64–76% of max heart rate) can improve sleep quality by reducing night waking and increasing stage 2 sleep.
Caffeine, alcohol, and nicotine in the afternoon or evening reduce sleep quality and duration.
Caffeine is a stimulant found in coffee, tea, and energy drinks. Afternoon or evening caffeine intake can make it harder to fall asleep and reduce total sleep by 30–60 minutes. Both timing and dose are important. For example, 107 mg of caffeine should be consumed at least 8.8 hours before bedtime, and 217.5 mg at least 13.2 hours before bedtime, to avoid negative effects.
Alcohol may help people fall asleep faster, but it reduces sleep quality. Even small amounts (about two drinks) delay REM sleep and shorten REM duration. Avoid alcohol within 4 hours of bedtime.
Nicotine stimulates the nervous and circulatory systems. It reduces sleep quality, duration, and ease of falling asleep. Stop nicotine use at least 4 hours before bedtime.
The sleep calculator can suggest the best times to stop caffeine, alcohol, and nicotine based on your wake-up time.
Poorly timed naps can make it harder to fall asleep and reduce night sleep quality. Napping less than 7 hours before bedtime can reduce sleep quality. Frequent or late naps may cause fragmented night sleep.
Published: 14.10.2025
Updated: 06.11.2025
Lassi Honkanen
physiotherapist
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