Why is it so good to sleep in rainy days?

Bai Juyi once wrote in "Inviting Zhang Yuzhong to Stay": "Muddy is not a day to travel, gloomy is a good day to sleep."

Many people will feel this way, especially when it rains. Once they fall asleep, they will fall into a deep sleep. Even if they wake up after a long sleep, they will feel weak and sleepy. Why is this?

Decreased sympathetic nerve activity

First of all, the air pressure decreases in rainy weather, and the oxygen content in the air decreases. At this time, people's sympathetic nerves will be inactive, parasympathetic nerves will dominate the body, and it is easy to get sleepy. In addition, due to the high humidity in rainy days, the moisture in the human body is not easy to be discharged from the body at this time, so besides being sleepy, you will feel heavy and listless.

White noise has played its role.

Secondly, something called "white noise" is at work. (ps: Why is it "white"? Because white light is a mixture of monochromatic lights with various frequencies (colors), this property of a signal with a flat power spectrum is called "white", and the signal is therefore called white noise. In contrast, other noise signals that do not have this property are called colored noise. )

Simply put, white noise refers to noise whose power spectral density is evenly distributed in the whole frequency domain. In nature, rain, running water, birds singing, waves beating against rocks, rustling of wind blowing through leaves and so on are all close to white noise.

We all find it difficult to fall asleep in a particularly quiet and noisy environment, but it is easy to fall asleep in a relatively slow and regular sound environment. Obviously, this is mainly related to our auditory system. When we sleep, our auditory system is in a standby alarm state, and we will notice the sudden sound and send out a signal to wake us up. White noise has the function of covering other external noises evenly, which produces a "shielding effect" and makes us sleep peacefully.

Therefore, white noise is called "natural lullaby" and is widely used in practice. For example, Queen Charlotte Hospital in London, England, in cooperation with medical equipment companies, first produced a white noise hypnotic device. As long as this white noise hypnosis is played in the child's bedroom for about 2 minutes, the baby who was originally restless and noisy in the bedroom will quickly and naturally fall into a sweet dream. In addition, there are special websites that provide "white noise" on the Internet to help people sleep better.

Monorhythmic stimulation

When rain falls on the ground, buildings and other objects, it will produce sounds. Usually, the sound of rain is not very loud, showing a small-big-small change process. When a person's cerebral cortex is stimulated by a single rhythm for a long time, it will cause deep inhibition, reduce the excitability of all parts of the body, and spread the influence of inhibition from the cerebral cortex to the subcortical part, thus causing drowsiness and making people fall asleep.

Such scenes are not uncommon, for example, the mother will gently and rhythmically pat the baby to sleep; When you enjoy the scenery outside by train and hear the train whistling rhythmically on the tracks, you will gradually feel tired; Besides, we listen to music to help us fall asleep before going to bed. Some people may say that the more you listen, the more excited you are. It's just that the style of the songs we choose is too cheerful or immersed in the songs. Try to read some boring English words. ......

Magic melatonin

Melatonin is an amine hormone produced by the pineal gland of mammals and humans, so it is also called pineal hormone. Melatonin has many physiological functions, such as anti-aging, immunomodulation, anti-tumor, and jet lag adjustment. One of its important functions is to promote sleep, so it has also been developed as a new hypnotic drug.

Because the secretion of melatonin has a circadian rhythm, research shows that the secretion of melatonin at night is about 5 ~ 10 times that during the day (this is because the pineal gland is controlled by the optic nerve), so the human body will also increase the secretion of melatonin in dim light, and we can understand why it is difficult for most people to fall asleep with the lights on. On rainy days, due to insufficient light and dark sky, people's melatonin secretion will increase, so sleepiness will also hit.

Evolutionary theory

Some experts speculate that this may be related to the evolutionary history of mankind. In ancient times, people lived in caves or thatched cottages, and when they slept at night, they would unconsciously worry about the intrusion of wild animals. Wild animals generally don't haunt in rainy days, so people will relax their vigilance and have a temporary sense of security in rainy days. Over time, although it has evolved for tens of millions of years, this habit has been inherited, which makes people instinctively relax in such weather.