1. Reduced sympathetic nerve excitability

When a person is extremely excited, we like to say that it is like getting a shot of chicken blood. In fact, this is the excitement of the sympathetic nerves in the body. When there is something outside When there is sufficient oxygen, the sympathetic nerves are easily excited. When the weather is sunny, plant photosynthesis is strong, oxygen is sufficient, and sympathetic nerve excitability is relatively high. However, when it rains, thick clouds block the sunlight, photosynthesis is weakened, oxygen content is reduced, and sympathetic nerve excitability is reduced, resulting in drowsiness. Low air pressure reduces external oxygen content, which is why some people with chronic obstructive pulmonary disease feel chest tightness and insufficient oxygen on rainy days.

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2. Increased secretion of melatonin

The pineal gland of the human body secretes a hormone related to sleep—melatonin . Melatonin can improve sleep quality, significantly reducing the number of awakenings during sleep, shortening the light sleep stage, and prolonging the deep sleep stage. Melatonin secretion is characterized by secretion at night or when the light is dim. On rainy days, dark clouds block the sunlight and the environment becomes dim. At this time, the secretion of melatonin from the pineal gland increases, making the body feel sleepy.

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3. Hypnosis by the sound of rain

Although the sound of rain is also noise, it is not an ordinary noise, the rustling sound of rain , its audio frequency is generally between 0 and 20 kHz. People are often very sensitive to the audio frequency in this range. We call this audio frequency "white noise". When the uniform sound of rain enters the brain, the brain will relax unconsciously and generate alpha waves. This state is very close to the state of the human brain during sleep. So, listening to the rustling rain is like listening to a lullaby. And "white noise" can also cover up other annoying noises.

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4. The negative ion content in the air increases

When it rains, the humidity of the air increases, and the friction between rain and air As well as thunder and lightning, negative ions are produced. Negative ions have a sedative and hypnotic effect, making it easier for people to fall asleep.

5. Reduced outdoor activities

Many outdoor things cannot be done on rainy days, and people can only stay indoors, leaving people with nothing to do. For people who have nothing to do, sleeping may be the best option.

The desire to sleep on rainy days is related to the long evolutionary process of human society. During the hunting period of primitive humans, they often hunted on sunny days and rested at night and on rainy days. After tens of thousands of years of evolution and survival of the fittest, this habit has been preserved and exists in human genes. On cloudy and rainy days, the light is relatively dark, giving people the illusion that it is night. The eyes send a signal to the brain (if it is not yet dawn, you can continue sleeping). However, this signal may be false. This is also due to the conditioned reflex of human beings' long-term evolution.