Shakespeare already told us, more than four centuries ago, that summer nights are more for love, bacchanal and worldly delight, than for sleeping. But the trickster fairies of his Summer night Dream (1595) are not the only ones to blame for enthroning Cupid and frightening Morpheus in this season: science attributes our summer insomnia to a host of factors. To begin with, the duration of sleep is shorter than in winter, as confirmed by research in Japan. “In summer there is a tendency to experience social jet lag, people go to bed later and it is difficult to tell children to go to bed when it is still daytime,” says Milagros Merino, specialist in clinical neurophysiology and president of the Society Spanish of the Dream (SES). “Minimum comfort conditions are needed to sleep, but we are having 31 and 32 degrees at midnight. With all the heat that the air conditioning equipment puts out, with which there is accumulated in the asphalt, it is infernal”, states the also coordinator of the neurological sleep disorders unit of the La Paz University Hospital in Madrid.
“Until we detect a significant drop in temperature, we don’t feel like sleeping,” explains Juan Antonio Madrid Pérez, professor of physiology at the University of Murcia and an expert in chronobiology, a discipline that studies biological rhythms. Naturally, body temperature drops one or two hours before going to bed to facilitate sleep and continues to drop while sleeping, something that makes it difficult to get hot, as explained by this expert who will publish in September Chronobiology. A guide to discovering your biological clock (Current Platform). In addition to sleeping less time, our sleep is of poorer quality. “It is more fragmented, more superficial. First because it is hotter, the number one enemy of sleep, but there is also more noise. In the cities, people go to bed later, more parties are organized”, adds Madrid Pérez, recalling that in Spain, festivals and popular celebrations are usually officiated in July and August. In fact, in places that are noisy at night, such as some areas in the center of Madrid, those without air conditioning have to choose between keeping the windows closed and not sleeping because of the heat, or opening them and not sleeping because of the noise. “City councils should look at this issue much more,” claims the specialist.
“In summer there is a tendency to experience social jet lag, people go to bed later”
Milagros Merino, president of the Spanish Sleep Society (SES)
A study published in 2021 described the summer sleep quality of 41 Shanghai households by monitoring air temperature, relative humidity, ambient noise level, and CO₂ concentration [el gas que exhalamos con la respiración y se acumula en habitaciones cerradas]. The researchers found that the most comfortable temperature and relative humidity of the air during sleep were 24.8 °C and 64%, respectively. They also found that higher air temperature and higher CO₂ level had a greater impact on men’s sleep, while noise level affected women more. In general, there are important differences between the sleep of men and women and, according to the SES, insomnia is more frequent in women, but research has focused disproportionately on men, as indicated by research from the University of California, leaving wide gaps on the specific sleeping difficulties of the female sex.
What we do know is that the elderly and children are the most affected by the heat. On the one hand, from the age of 60 or 65 there is a greater sensitivity, indicates Madrid Pérez. “One degree of nighttime temperature rise twice as much affects older people than young people,” she points out. “Everything ages: the tissues, the brain and one of the brain structures that controls sleep and thermal adjustment, the hypothalamus,” explains Merino. At the opposite vital extreme, during the first months or years of life, the same distortion is produced by the immaturity of that brain region. Therefore, “babies and the elderly are the ones who notice extreme temperatures the most,” he warns. “To sleep we need to cool down the body,” emphasizes Madrid Pérez, with which people who are overweight or obese also multiply their chances of insomnia, already high due to their greater tendency to obstructive sleep apnea. “Their greater amount of fat makes it difficult to lose heat and they have to sweat to thermoregulate,” the expert emphasizes.
Every night, normal sleep occurs in several cycles divided into four or five phases (according to various classifications), which the summer also affects differently. “When you sleep more superficially, like when it’s hot, we lose phase 3 sleep,” explains Madrid Pérez, who contextualizes the stages. On the one hand, there is REM sleep [acrónimo en inglés de movimientos oculares rápidos], during which daydreams take place and which occurs mainly around 5 or 6 in the morning. On the other, non-REM, calmer and during which we do not move our eyes under the eyelids, which is subdivided into three phases: 1 or superficial, like a nap from which we wake up without knowing if we have fallen asleep; 2, which begins to be restorative; and the most affected by high temperatures, phase 3 or deep sleep, “the one that recovers your body and your brain the most,” says the expert. “When you wake up too early, REM sleep is also affected,” he adds.
To heat and noise, light is added as a problem. The fact that in summer the days are longer and we have more hours of light alters our biological rhythms, specifically the so-called circadian rhythms. This concept comes from the Latin (circa means “around” and ten, “day”) refers to the cyclical changes that many biological variables experience during the 24 hours, such as our temperature or blood pressure. They are variations regulated by hormones and associated with some environmental stimulus, such as the day-night cycle. In mammals, a central circadian clock in the hypothalamus —“equivalent to two grains of rice of neurons”, in the words of Madrid Pérez— tunes these physiological rhythms with the light-dark cycle.
“A degree of rise in night temperature affects the elderly twice as much as the young”
Juan Antonio Madrid Pérez, University of Murcia
One of the hormones that changes the most in the dark is melatonin, produced by the brain’s pineal gland when it determines that its night has come. “In chronobiology, we call it subjective night because it is what marks your biological clock, not necessarily your wristwatch,” explains Madrid Pérez, who is also responsible for Cronolab, the chronobiology laboratory at the University of Murcia. Said clock regulates that subjective night based on the personal pattern of exposure to light and darkness or dinner times, continues the expert. “Social media activity is also playing a role in delaying it,” he adds.
And genes also influence: “We have individuals who are morning, others evening and others in between. A morning paper is going to start its biological night two, three or four hours earlier than an evening paper,” she considers. “When that night comes, your biological clock stimulates the pineal to secrete melatonin if it is simultaneously dark. If we turn on the light at night, it will not happen because it needs both conditions simultaneously”, he points out. But in summer, everything is fleas: “by 6:30 or 7 in the morning we already have light and melatonin drops, while at night we work until late and with artificial light”. That is, we sleep for less time and produce less of the hormone that would favor our sleep and the recovery of the body.
top tips
So how do we lure back the Greek god of dreams? “What we have to do is boring,” anticipates Merino. On the one hand, there are general measures valid in any season:
Consistency is key: “We need regular schedules that our brain can predict and to which it can adapt over time,” emphasizes Madrid Pérez. Ideally, our wake-up or bedtime shouldn’t change more than an hour to an hour and a half from one day to the next, including weekends.
Twilight before going to bed: Subjective nighttime is altered by the amount of light present before sleep, and darkness stimulates melatonin production, so dim bright lights and even consider wearing a sleep mask. “If you receive light from mobile phones, tablets or computers, your brain interprets that it is still daytime because it is the light that most stimulates your biological clock. If we need to use screens, let it be low-intensity two hours before bed,” advises Madrid Pérez.
Welcome daylight: Unlike at night, exposing ourselves to light in the morning helps regulate circadian rhythms, so going outside is recommended. However, light that filters in early can wake us up prematurely.
Prepare sleep during the day: “We can’t get to bedtime with our brain working at 100%, after answering emails or working late, we need to disconnect,” warns Madrid Pérez. How long? “It depends on the person: some need an hour, others two.”
Proper aeration: The CO₂ accumulated in closed rooms, especially if they are small or shared, can affect sleep. When this gas that we exhale increases or decreases oxygen, the brain sends out signals to wake up, as occurs in patients with apnea, exemplifies Madrid Pérez. “They are micro-arousals of a few seconds that we do not perceive, but they do affect the depth and fragmentation of sleep.” Let’s ventilate, then, before sleeping and during the night.
Avoid exciting and copious dinners: “We should avoid consuming alcohol, tobacco or caffeinated beverages late in the day. If we notice the effect of coffee, the ideal is that the one after eating is the last”, suggests Merino. Dinner, two or two and a half hours before going to bed and not abundant because digestion hinders deep and restful sleep.
Exercise at the right times: “You don’t have to do very stimulating activities towards the end of the day,” reiterates Merino. “Exercise in the morning is more recommended, it will keep you in a state of greater relaxation and mental and physical balance throughout the day. Also, if you do it in the street, it will mean exposure to natural light in the morning, which is very healthy”, explains Madrid Pérez. “Intense exercise two hours before bed, ruled out; moderate exercise or walking, perfect”.
And specifically for the summer, specialists recommend the following:
Improve bedroom temperature: “We need to cool the brain to sleep,” says Madrid Pérez. “If the room is not below 26°C, there is no choice but to use air conditioning or a fan,” she considers. It may help to open the windows and draw the blinds only early or after dark.
warm shower: Although it may be counterintuitive, three specialists from the Swinburne University of Technology, in Australia, explained in The Conversation why to stay cool in summer it is more effective to shower with water at around 33⁰C than with cold water. The latter reduces blood flow to the skin and, therefore, heat loss, which leads to an unwanted heating of our body core and, minutes later, the hot flash returns. Conversely, the warm sensation on the skin causes skin vasodilation, which increases the loss of body heat and greater comfort after a few minutes.
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