Have some questions about sleep?

Why do we need sleep? What is sleepiness?


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  • Please note that the information in this article is intended for adults regarding sleep unless otherwise stated.

It seems that there are two main mysteries at the center of basic sleep research: why do we need sleep, and what is sleepiness?

​​Why do we need sleep?

By sleeping, we are performing the regular upkeep of our brains. But the idea that we rest while we’re asleep may in fact be a misconception. This is because our brains continue to be active even while we’re sleeping. Observing the nerve cell activity in the cerebral cortex of a sleeping person, we see that there is almost no difference from its activity while the person is awake. Because the nerve cells remain active, it seems a bit of a stretch to say that the brain is resting.

​​By the way, all animals on Earth that have nervous systems sleep.​ 

Because we lose consciousness while we sleep, cutting us off from the outside world, sleep can be a very dangerous state for wild animals. An animal that didn’t sleep would have a significant advantage over others, but no such animal has yet been discovered. Therefore, it is thought that sleep must serve a crucial purpose that outweighs the risk it poses. But the specific details as to why we need sleep have yet to be explained.

​​●What is sleepiness?

Our brains are always keeping track of how long we’ve been awake. This varies from person to person, but most people become sleepy after 14 to 15 hours have passed since getting up. Our brains also keep track of the cumulative amount of time we’ve been awake over the past several days, and we become sleepy when that cumulative amount of time grows too long.

​​The only way to rid ourselves of sleepiness is to sleep. If you are tired from overwork, just lying down and remaining still might seem like it would be helpful—but if you don’t actually sleep, you will never reduce your sleepiness.

However, information such as where exactly in the brain the sleepiness mechanism is located and how it works, as well as how the brain keeps track of time, has yet to be found, and is being researched even now.

A recent theory suggests that phosphorylation, a chemical reaction with proteins in the brain, may play a role in making us feel sleepy.

Reference material: Wang, Zhiqiang, et al. “Quantitative phosphoproteomic analysis of the molecular substrates of sleep need.” Nature 558.7710 (2018): 435-439.

Kim, S.J., Hotta-Hirashima, N., Asano, F. et al. Kinase signaling in excitatory neurons regulates sleep quantity and depth. Nature 612, 512–518 (2022).

Zhou, R., Wang, G., Li, Q. et al. A signaling pathway for transcriptional regulation of sleep amount in mice. Nature 612, 519–527 (2022).

About Dr. Yanagisawa

Masashi Yanagisawa

Born in Tokyo in 1960, Masashi Yanagisawa completed his medical doctorate at a graduate school in the University of Tsukuba and is a member of the National Academy of Sciences. In 1987, he discovered the vasoconstricting peptide endothelin while in graduate school, and in 1988 he discovered orexin, a neuropeptide that regulates sleep and wakefulness. At 31 years of age, Professor Yanagisawa came to the United States, where he presided over laboratories at the University of Texas and at the Howard Hughes Medical Institute over the span of 24 years. He founded the International Institute for Integrative Sleep Medicine (WPI-IIIS) in 2012, establishing it under the Japanese Ministry of Education, Culture, Sports, Science and Technology’s World Premier International Research Initiative. Professor Yanagisawa founded S’UIMIN Inc. in 2017 and currently acts as CEO.

He supervised the provision of sleep-related information during the development of Pokémon Sleep.

He was awarded a Japanese Medal with Purple Ribbon in 2016, received the Asahi Prize and the Keio Medical Science Prize in 2018, was named a Person of Cultural Merit in 2019, and received the Breakthrough Prize in 2023.