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StillnessHealth fact

Morning light is one of the main signals that sets the body's internal 24-hour clock, known as the circadian clock. Bright outdoor daylight is usually a far stronger signal than ordinary indoor lighting.

Circadian research

HealthMind

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Special light-sensing cells in the retina, separate from the ones we use to see shapes and colours, report light levels to the suprachiasmatic nucleus, a tiny area just above the point where the optic nerves cross, which acts as the body's master clock. It sets the timing of many daily rhythms, including the release of melatonin, the hormone that helps bring on sleep. Measured in lux, the unit of brightness, outdoor daylight ranges from a few thousand on a dull day to about 100,000 in full sun, while indoor lighting is often only a few hundred. Light early in the day tends to shift the clock earlier and light late in the evening shifts it later; how much depends on timing, brightness, duration and a person's existing rhythm.

Türkçe oku
Sabah alınan ışık, vücudun sirkadiyen saat denen 24 saatlik iç saatini ayarlayan başlıca işaretlerden biridir. Dışarıdaki parlak gün ışığı, sıradan iç mekân aydınlatmasından genellikle çok daha güçlü bir işarettir.

Retinada, şekil ve renk görmemizi sağlayan hücrelerden ayrı, ışığa duyarlı özel hücreler bulunur. Bunlar ışık düzeyini, görme sinirlerinin kesiştiği noktanın hemen üstündeki küçük bir bölgeye, suprakiazmatik çekirdeğe bildirir. Vücudun ana saati sayılan bu bölge, uykuya dalmayı kolaylaştıran melatonin hormonunun salgılanması dâhil birçok günlük ritmin zamanlamasını belirler. Işık şiddeti lüks birimiyle ölçülür: Dışarıdaki gün ışığı kapalı bir günde birkaç bin, güneşli bir öğlede yaklaşık 100.000 lükse ulaşır; iç mekân aydınlatması ise çoğu zaman birkaç yüz lükste kalır. Günün erken saatlerinde alınan ışık iç saati öne, akşam geç saatte alınan ışık ise geriye kaydırır; ne kadar kaydıracağı zamanlamaya, parlaklığa, süreye ve kişinin mevcut ritmine bağlıdır.

Sources and further reading

Background reading: Circadian rhythm
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While you sleep, the brain strengthens what you learned during the day. This process, called memory consolidation, works both on facts, such as names and dates, and on skills, such as playing a piece of music. After a short or broken night, the brain holds on to new learning less well.

Memory consolidation is the slow process that turns a fresh, fragile memory into a lasting one, and much of it happens during sleep. In 1997 the German researchers Werner Plihal and Jan Born showed that different parts of the night help different kinds of memory. Twenty healthy men learned lists of word pairs and practised mirror tracing, drawing a shape while watching only its reflection. Recall of the word pairs improved most over the first half of the night, when deep, slow-wave sleep dominates, while the drawing skill improved most over the second half, when REM sleep, the stage of vivid dreams, lasts longer. Researchers think that during sleep the brain replays the day's experiences and moves new memories from temporary storage in the hippocampus, deep in the brain, into longer-lasting networks in its outer layer, the cortex.

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Some benefits of exercise begin on the day you do it, according to US guidance promoted by the CDC. After a single session of moderate or vigorous activity, such as a brisk walk, blood pressure can fall, anxiety can ease, and sleep that night and some kinds of thinking can improve.

The CDC (Centers for Disease Control and Prevention) is the United States' national public health agency, founded in Atlanta in 1946. Its advice on exercise follows the Physical Activity Guidelines for Americans, whose second edition, published in 2018, drew on a review of the previous decade's research. Earlier advice had focused on preventing disease over the years; the new edition also listed benefits people can feel at once. Besides easing anxiety and lowering blood pressure, a single session helps the body respond to insulin, the hormone that moves sugar from the blood into cells. Most of these effects grow with regular activity, while others, such as fitness, strength and a lasting fall in blood pressure, take weeks or months. The guidelines recommend at least 150 minutes of moderate activity a week.

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Regular physical activity protects the heart, lifts mood, eases anxiety and improves sleep, according to the World Health Organization. These benefits do not depend on losing weight: people of every shape and size gain them, even when nothing changes on the scales or in the mirror.

The World Health Organization counts any movement that uses energy as physical activity: walking, cycling, sport, active play, housework and physical work. Its 2020 guidelines advise adults to do 150 to 300 minutes of moderate activity a week, such as brisk walking, or 75 to 150 minutes of vigorous activity, such as running, plus muscle-strengthening exercise on at least two days. In 2022 about 31% of adults worldwide, some 1.8 billion people, fell short of that. Weight is a poor guide to these benefits. The US Physical Activity Guidelines state that they are generally independent of body weight, and a 2014 analysis of ten studies found that fit people who were overweight or obese had death rates similar to fit people of normal weight, while unfit people had about twice the risk, whatever their weight.

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Sleeping soon after you learn something helps you remember it. In a 2012 study, people who learned word pairs in the evening and slept soon afterwards remembered more 24 hours later than people who learned them in the morning and spent the day awake first.

The study was led by Jessica Payne of the University of Notre Dame, with colleagues including Robert Stickgold of Harvard Medical School. Volunteers learned pairs of words at 9 am or 9 pm. Twelve hours later those who had slept remembered more, mainly because a day awake had worn away memory for pairs of unrelated words, which are harder to link in the mind. After 24 hours everyone had had one night's sleep and one day awake, only in a different order, and those who slept soon after learning still remembered more. Memory also faded more slowly across a day awake when a night's sleep had come first, which suggests that sleep steadies new memories against the interference of waking life. It is one reason sleep researchers advise against giving up sleep to cram for an exam.

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Sleep is part of learning, not a pause from it: while you sleep, the brain goes back over what you practised during the day and strengthens it. That is why a new skill often runs more smoothly the next morning than it did the evening before.

In a 2002 study at Harvard Medical School, led by Matthew Walker, volunteers learned to type a short number sequence as quickly and accurately as they could. After a night's sleep they were about 20 per cent faster, with no loss of accuracy, while the same number of hours awake brought no significant gain. The size of the improvement was linked to how much light sleep, known as stage 2, people had, especially late in the night. Animal studies from the 1990s suggest how this might work: in rats, groups of brain cells that were active while the animals explored became active together again during sleep, as if the day were being replayed. It is one reason why an extra tired hour at the desk may help less than a full night's sleep.

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