Night owl or lark or in-between: everyone operates on a circadian cycle referenced to day and night outdoors. There are two main ways to place an individual on this dimension: physiological and behavioral.
Physiological specification of early, middle, or late types can be made by taking samples of saliva throughout the evening, up through bedtime, and measuring the amount of melatonin. The late types will start producing melatonin hours later than the earlier types, and we can place them precisely on the scale of circadian phase position.
The behavioral way to assess yourself as night owl or lark or in-between is by daily sleep log or a questionnaire that asks how well you perform certain activities (or would prefer to perform them) across the day. It is remarkable how these behavioral measures of chronotype correlate strongly with the physiological measures.
Chronotype tells us when a person’s circadian day and night are occurring. This information can help to optimize the schedule of light therapy, melatonin administration, or hours outdoors or in a dark bedroom, to facilitate therapeutic shifts of circadian rhythms, earlier or later. In other words, we want to specify the treatment schedule in circadian time, rather than the time of day according to the clock on the wall, which can easily provide a mismatch to a person’s inner clock.