'Sleep separately': A sleep specialist's guide to better rest

"My wife and I sleep in separate beds. We share the same room, but we placed a storage unit between the beds to keep them completely apart. My sleep quality improved after we made that change."

Kim Dong-gyu, an ear, nose and throat specialist and public relations director of the Korean Sleep Society, named "independent sleep" as the single most important condition for a good night's rest. Couples do not necessarily need separate rooms, he said, but using individual beds with space between them — so neither partner is disturbed by the other's tossing, body heat or snoring — makes deep sleep significantly easier to achieve.

In an interview Wednesday, Kim said that even when couples share a room, splitting the beds reduces the chance of being woken by a partner's movements. "I also had my two children sleep in their own beds from an early age," he added. Kim uses the bed strictly for sleeping — no smartphones, no reading, no work. The goal, he explained, is to train the brain to associate lying down with falling asleep.

"I can look at my phone before I get into bed, but once I lie down, I stop," he said. "It is also a bad habit to lie in bed waiting for sleep to come when you are not yet drowsy." When he reads before bed, he does so outside the bed — in a dimly lit space, with a print book. Only when the words start to blur and drowsiness sets in does he move to the bed.

"The bed should simply be a place for sleeping," he said. "Once you lie down, it is best to fall asleep within 15 to 30 minutes. My children generally fall asleep within 30 minutes of getting into bed." Before sleep, he avoids bright overhead lights and the glow of smartphones. Even if white light is used during the day, switching to warm-toned LED lighting with a lower color temperature at night is better, he said — bright light entering the eyes suppresses melatonin secretion and can delay the onset of sleep.

Kim also places great importance on a cool sleep environment. "Within 90 minutes of falling asleep, your core body temperature needs to drop by 1 to 2 degrees Celsius for deep sleep to follow," he said. "Using a mattress with good ventilation and cooling bedding that allows body heat to escape efficiently is extremely important."

"The reason tropical nights in summer make it hard to sleep is that core body temperature does not fall far enough," he said. "The first 90 minutes after you fall asleep are what I call the 'golden window' — they determine the quality of everything that follows."

Kim explained the mechanics of sleep through the interaction of two curves: hormones — specifically steroids and melatonin — and fatigue. Right after waking in the morning, he said, fatigue is low, steroid secretion rises and melatonin is suppressed, producing a state of alertness. It is the pleasant feeling of a fresh morning.

That sense of alertness peaks around midday, then begins to decline in the afternoon as melatonin's influence grows and the circadian rhythm dips. The common habit of drinking coffee after lunch, he noted, helps sustain alertness and slightly delays that dip. "This is what the circadian rhythm looks like," Kim said.

Fatigue works differently. "Fatigue accumulates continuously from the moment you open your eyes and start moving," Kim said. "In the afternoon, the circadian rhythm falls while fatigue keeps climbing, widening the gap between the two curves. When that gap is at its greatest, a powerful wave of drowsiness arrives and sleep begins — and that is also when a cup of coffee can help you stay alert if you need to."

"Just before you fall asleep, fatigue is at its highest and the circadian rhythm is at its lowest — that is the natural moment for sleep," he said. "When you wake up afterward, fatigue drops to its minimum and a new day begins."

Kim addressed patients who say they can only fall asleep after drinking alcohol. "Alcohol causes fatigue to spike sharply — you talk more, think more and move around more," he said. "That does bring on sleep, but the problem is that when fatigue has risen so steeply, you wake up again quickly. Alcohol may help you fall asleep in the short term, but it actually interferes with deep, restorative sleep."

He said smartphones and video content disrupt the balance between fatigue and the circadian rhythm. The light from screens suppresses melatonin secretion, slowing the natural decline of the circadian rhythm. At the same time, the brain keeps processing new information from video and audio, driving fatigue even higher. Falling asleep while watching a phone, he said, makes it difficult to achieve truly restful sleep.

"People who say they can only fall asleep with a smartphone or podcast playing have simply trained their brains to work that way," Kim said. "They may fall asleep, but the quality of that sleep is likely to be worse than if they had watched nothing at all."

Print books are a better option than smartphones. Because there is no screen glare or rapidly changing images, reading quietly lowers the brain's level of arousal. "Books are good for building knowledge and they also help bring on sleep," Kim said. "Many people read precisely because they want to fall asleep."

"Older patients sometimes come in and say they are worn out but cannot fall asleep," Kim said. "That is because declining melatonin levels and muscle loss are working together."

"Melatonin secretion begins to decrease after the age of 40 and drops sharply after 60," he said. "As the body's ability to sustain sleep weakens, people may find it harder to fall asleep or may wake frequently in the early hours. Particularly in older adults, the loss of muscle mass also affects how fatigue is perceived."

Even when fatigue-related substances such as adenosine accumulate in the body, Kim explained, a reduced ability to transmit those signals to the brain as tiredness means the body may feel spent while the brain does not register enough drowsiness to trigger sleep. "Quite a few elderly patients tell me they went hiking or walked laps around a track to try to sleep, and still could not," he said. "Aching legs and the brain recognizing the kind of fatigue needed for sleep are two different things."

"For insomnia in older adults, combining strength training with aerobic exercise — rather than relying on aerobic exercise alone — can be helpful," Kim said. "However, late-night running should be avoided, as it raises both mental and physical arousal and can interfere with sleep."

"Running is better done in the morning or during the day; light strength training in the evening is preferable," he said. "I do not recommend high-intensity aerobic exercise late at night for people with sleep disorders." Kim assessed plant-based melatonin supplements sold commercially as having limited effectiveness, saying products classified as food — rather than medicine — are not sufficient to treat actual insomnia or sleep disorders.

"People who are deficient in melatonin do need to supplement it, but plant-based melatonin products are not the same as pharmaceutical preparations," Kim said. "If insomnia persists, rather than self-medicating with over-the-counter products, you should see a sleep specialist and receive a proper prescription."

Kim pointed to REM sleep as the reason humans chose to sleep through the course of evolution. "For early humans, falling into deep sleep at night was a dangerous act — it meant exposure to predators, venomous creatures and attacks from other people," he said. "Yet they did not give up sleep because the benefits of REM sleep — organizing the day's information and experiences into long-term memory and recovering from emotional damage — were necessary for survival."

"During REM sleep, the brain sorts newly acquired information and stores it as memory," Kim said. "It also processes negative emotions accumulated during the day — worry, anger, anxiety. Without enough sleep, the irritability, depression and anger felt on Wednesday are not fully resolved. A good night's sleep allows emotional damage to recover to some degree, so you can carry on with daily life."

He sees the frequent co-occurrence of insomnia and depression as closely connected to this mechanism. Sleep deprivation makes emotional regulation harder, and anxiety and depression in turn disrupt sleep — creating a vicious cycle.

"Humans chose sleep even at the risk of danger, in order to survive," Kim said. "Yet modern people are cutting back on sleep for good schools, good jobs and social success. It is an ironic situation — the very thing chosen for survival is now being sacrificed for survival."

"Only a tiny fraction of people are genetically able to recover on minimal sleep," Kim said. "The lifestyle of a few individuals who succeeded on little sleep should not be treated as a formula everyone must follow."

Maintaining a consistent wake-up time matters more than a fixed bedtime, Kim said. Even if a person goes to bed late on Wednesday, waking at the usual time the next morning allows drowsiness to arrive naturally the following evening and helps restore a disrupted circadian rhythm. "Someone who normally falls asleep at midnight and wakes at 7 a.m. should still get up at 7 a.m. even if they did not fall asleep until 3 a.m.," he said. "When fatigue builds from the sleep deficit, drowsiness will arrive earlier than usual that night and the rhythm will reset."

Kim also advised against trying to catch up on sleep in large amounts on weekends. However, if sleep debt has already accumulated, some catch-up is better than none at all. Naps should be kept to between 30 minutes and one hour to minimize the impact on nighttime sleep.

"The best approach is to sleep six to eight hours regularly every day," Kim said. "If you have realistically fallen short on sleep, catching up may be unavoidable — but it is not something I would recommend as a general practice."