Sleeplessness at night and sleepiness during the day are unrelated aside from their commonality during the summer months.
Normally your body temperature drops as you fall asleep. This can be difficult to do when the surrounding air is close to or above this temperature. Likewise women on their period often have difficulty sleeping because this elevates their body temperature. If you must sleep in a warm area taking a shower before going to bed can help: a cold shower cools the body directly while a hot shower tricks the body into lowering its temperature.
Dehydration is a common cause of insomnia. Mild dehydration interrupt your Circadian rhythm, the daily pattern of body temperature and melatonin changes that control your alertness. More severe hydration can dry mucous membranes around they eyes, nose, and mouth, causing irritation. Humid weather can increase dust mites and mold further irritating these membranes. The warmer air is the more moisture it can carry, increasing these factors: The air can carry twice as much water vapor at 95 degrees F as it can 75 degrees F.*
As far as we know, heat can only keep you from falling asleep. So, why do you feel sleepy on a hot afternoon? It's not the heat, it's the time. Your Circadian rhythm takes a dip in the early afternoon regardless of whether it's hot out or not.
Many cultures in hot climates have a tradition of taking naps in the early afternoon, but this is not directly caused by the heat. Instead, it keeps people out of the sun when they are most in danger of heat exhaustion and ultraviolet light exposure. This nap almost universally happens right after lunch, especially in cultures where lunch is the main meal of the day. After eating your body has a brief drop in glucose levels from all the insulin being released to handle the influx of food. This makes you drowsy.
While thought of as a summer activity, siestas take place year-round in most Spanish-speaking countries. Spain in particular is noted for its big lunches and late nights making the siesta a practical activity regardless of the weather.
*Technically the air canít ďholdĒ water as the water molecules are not attached to the air molecules. Instead the water vapor floats around with the oxygen, nitrogen, and other gasses in our atmosphere.
Posted 3170 day ago