This refers to the body's "circadian rhythm" which operates on a roughly 24 hour cycle, separately from the actual time in the outside world. It's also referred to as our "body clock". Our "body clock" is affected by light and dark, and so is normally synchronised with day and night times where we are in the world. When we move quickly to a different longitude - as in flying from Europe to the US, for example - our body clock continues to operate for some time on the local time in Europe. Thus, we may arrive in Los Angeles, where it's 4 PM, but our "body time" is telling us that it's midnight [London time] - and we should be in bed asleep! It will take several days of experiencing the changing times of daylight in Los Angeles for our body time to re-synchronise with local time. This phenomenon is popularly called "jet lag".
