For one reason or another, there are increasingly more night owls nowadays than ever before. Some people have to work on their night shits; some prefer to work or play until late at night; others are simply internet addicts. However, no matter what the reason, staying up late habitually is particularly harmful, and much more so even than smoking, to one's physical wellbeing.
According to traditional Chinese medicine as well as western common sense (and recent modern medical studies), it is true, as Benjamin Franklin has said, that 'early to bed, and early to rise, makes a man healthy, wealthy, and wise.' In fact, the best bedtime is between 9:00-11:00 pm to 5:00-7:00 am every night. One, if not the, most important reason for this is very simple: your immune system begins to detox around 9:00 pm every day, while your liver cannot detox during its fixed timeframe between 11:00 pm and 1:00 am unless you sleep on bed. If you stay awake at night for too long and too frequently, the result can be quite disastrous to your liver, heart and almost your whole immune system.
That is to say, if you really want to yangsheng, DON'T be a night owl whenever possible!