You can move your bedtime by two hours in a single night. Your circadian clock may move only a fraction of that.

There is no simple answer to how long it takes to shift your body clock. Changing the time you sleep and changing your circadian timing are two different things, and they do not necessarily happen at the same speed.

You can decide to go to bed at ten instead of midnight tonight. The clock that helps determine when your body is ready for sleep does not move simply because the time on the clock has changed.

What it means for the clock to move

When researchers talk about your body clock, they do not mean the hour you choose to go to bed. They are referring to an internal timing system. Researchers cannot see that timing directly, so they use measurable changes in the body to track it. One of the most common is the evening rise in melatonin.

Researchers call this rise dim-light melatonin onset, or DLMO. Researchers use its timing as a marker of circadian phase, which simply means where you are in your internal 24-hour cycle.

DLMO is measured under controlled conditions and is not something you can check at home. When researchers report a shift in circadian timing, they are describing a change in this internal marker, not simply a change in someone’s sleep schedule.

How quickly it can actually move

A 2005 study gives a useful example of the difference between moving a sleep schedule and moving circadian timing. Charmane Eastman and colleagues had people gradually move their sleep earlier in preparation for eastward travel. One group moved their sleep one hour earlier each day and another moved it two hours earlier each day. Both groups received several hours of intermittent bright light after waking, timed to help move their circadian timing earlier.

After three days, the first group’s sleep schedule had moved three hours earlier, but their circadian timing had shifted about 1.4 hours. The second group’s sleep schedule had moved six hours earlier, but their circadian timing had shifted about 1.9 hours. Despite the much larger change in sleep schedule, the difference in circadian shift between the two groups was small and not statistically significant.

When the schedule moves faster than the clock

The same study showed what happens when the schedule outruns the clock. The group advancing two hours a day ended up trying to sleep on a timetable their internal clock had not caught up to.

That gap between your sleep schedule and your circadian timing is called circadian misalignment. In 1995, Derk-Jan Dijk and Charles Czeisler showed that how well you sleep depends not only on sleep pressure, the drive to sleep that builds the longer you are awake, but also on when that sleep occurs relative to your circadian phase. Sleep at an unfavourable circadian time tends to be shorter and more fragmented. Sleep pressure and circadian timing both influence how well you sleep.

This same mismatch is central to jet lag. As Waterhouse and colleagues described in 2007, much of the disruption comes from a schedule that has jumped to a new time while the body clock is still keeping the old one.

Earlier and later are not the same

Earlier and later may be the same distance on a clock, but they are not equivalent to your circadian system. Shifting your body clock later tends to happen faster than shifting it earlier. The same pattern appears in travel: circadian re-entrainment is generally faster after westward travel, a later shift, than after eastward travel, an earlier one.

The human circadian cycle averages slightly longer than 24 hours, which is one possible explanation for this asymmetry. Light can also move the clock in either direction depending on its timing.

When the clock does not fully follow

Sometimes the clock does not fully catch up. The clearest example is permanent night work, where people hold the same overnight schedule for months or years.

In 2008, Simon Folkard reviewed studies that measured melatonin rhythms in night workers. The studies looked at whether the timing of those rhythms had shifted to match their work schedule. Fewer than three in a hundred permanent night workers showed a full shift, and fewer than one in four showed a substantial shift. Keeping a schedule, even for years, is not the same as moving the clock to match it.

So how long does it take?

The research does not give us a reliable number of days for shifting your body clock. Studies can measure how much circadian timing changes under particular conditions, but those results are group averages, not a rate that can be applied to everyone.

The findings help us understand how the clock shifts, but they cannot predict exactly how long your own shift will take, because changing a sleep schedule and changing circadian timing are not the same.