Timing and temperature
Led by Shahab Haghayegh, a Ph.D. candidate in the department of biomedical engineering at The University of Texas at Austin (UT), the study, which was published in Sleep Medicine Reviews, found that taking a bath before bed can help you sleep better if you use the right water temperature and take your bath at the right time.
The scientists analyzed previous studies that linked three factors: Bathing, water temperature and sleep quality.
After reviewing 5,322 studies, the researchers found that bathing in water at temperatures between 104 to 109 F (40 to 43 C) helped participants get quality sleep.
Ideally, you should soak in a bath one to two hours, or 90 minutes, before you turn in for the night. Bathing for this long and at this temperature may help you fall asleep at least 10 minutes faster than you would normally do.
Sleep and circadian cycles
The study looked into how body heat is linked to your ability to fall asleep.
Studies have found that sleep and the body’s core temperature are determined by the circadian clock.
During the late afternoon or early evening, your body is at least two to three degrees higher than your normal temperature. It goes down while you're asleep.
When it's almost bedtime, the average person experiences a 0.5 to 1 F (0.3 to 0.6 C) drop in body temperature. This reaches the lowest level from the middle and later span of nighttime sleep. As your body gets ready to wake up, the temperature starts rising again.
Taking a hot bath or shower helps stimulate your body’s thermoregulatory system. This promotes blood circulation from the internal core to the peripheral sites in your hands and feet. All this can help remove body heat and lowers your body temperature.
Taking a hot bath at night is based on the core body temperature fall that signals your pineal gland to induce the production of melatonin. This usually happens from 10 p.m. to 11 p.m. for the average person each night.
However, the timing may change if you're an early bird, a night owl or an insomniac. It is believed that timing your bathing works because after you raise your body temperature over 100 F (38 C), it then has to fall, which is like the natural decrease.
Note that bathing in cold water can make your body temperature drop lower, which can make you feel more alert instead of helping you doze off.
If a bath or a shower right before bedtime isn't your cup of tea, try other natural sleep-promoting supplements like lavender, magnesium, melatonin or valerian root.
Bath tips for better sleep
Baths affect your body differently than showers.
A bath changes your body temperature faster because it is a stimulus that surrounds your body. Taking a bubble bath forms a layer of insulation around the body, which keeps it warmer longer. Since you're lying down in a tub, your body relaxes in a different way than if you take a quick shower. (Related: What are the best teas for sleep?)
Dr. Jianghong Liu, a professor at the University of Pennsylvania School of Nursing, warned that you shouldn't soak in the tub too close to bedtime since your body won't have enough time to cool down. This can make it harder to fall asleep.
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