Are you a nightowl?

TCM considers regularly going to bed late to be an unwise practice.   The reason for this is that nighttime is Yin and daytime is Yang.  Yin is a time of rest and Yang is a time of activity. 

The body regenerates Yin most efficiently when the sun is down.  If you go to sleep after the sun has been down for many hours you lose the most potent restorative effect of this Yin time of day as peak Yin occurs at midnight.  For example, from 11 PM to 1 AM the Gallbladder is considered to be most active and from 1 AM to 3 AM the Liver is most active.  If you remain awake through these hours, you prevent these two organs from regulating and detoxifying Blood.  This could cause symptoms of irritability, stiffness, vision problems, dizziness, fatigue, and reduce immunity.

Conversely, if you sleep too late in the morning when the sun has already risen for several hours, these last few hours also do not give you as much Yin as when the sun is down.  This is because Yang is increasing as the sun rises.   Yin is easy to lose (through overwork and stress), but hard to regenerate and so we would be wise to take advantage of the natural cycle of the waxing and waning of Yin and Yang in order to get the very best quality sleep.   It is well known that people that routinely work the night shift will suffer poorer health over the long term.