No. 1: Lack of sleep.
It’s considered macho in the USA to get by with a minimum of sleep and hold down two or three jobs, simultaneously.
In the UK, Margaret Thatcher famously was a light sleeper.
This is stupid.
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Sleep is a great healer. Conversely, lack of sleep affects the mind and body. Thoughts become more negative and the body becomes less energised. Wounds heal more slowly. Lines form on the face. Bags form under the eyes. The skin loses tone.
Not everyone can get by on 8 hours; some people need more.
It is perverse to deprive the body of what it needs to operate well. I propose that lack of sleep, over time, can lead to serious illness e.g. depression and cancer.
It may also cause you to operate in ‘zombie’ mode; you make poor decisions and then get more upset than you need to because of their consequences. Why? Because you’re sleep-deprived!