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.

REFERENCES

Reviews

Roenneberg T, Merrow M. Entrainment of the human circadian clock.  Cold Spring Harbor Symposia on Quantitative Biology 2007;72:293-299.
Terman M, Terman JS. Light therapy. In: Principles and Practice of Sleep Medicine (5th ed.), Kryger MH, Roth T, Dement WC, Eds. St. Louis, Elsevier/Saunders, 2010, pp. 1682-1695.

Studies

Horne JA, Ostberg O. A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms. International Journal of Chronobiology 1976;4:97-110.
Kerkhof GA, Van Dongen HPA. Morning-type and evening-type individuals differ in the phase position of their endogenous circadian oscillator.  Neuroscience Letters 1996;218:153-156.
Katzenberg D, Young T, Finn L, Lin L, King DP, Takahashi JS, Mignot E. A CLOCK polymorphism associated with human diurnal preference. Sleep 1998;21:569-576.
Duffy JF, Dijk DJ, Hall EF, Czeisler CA. Relationship of endogenous circadian melatonin and temperature rhythms to self-reported preference for morning or evening activity in young and older people. Journal of Investigative Medicine 1999;47:141-150.
Roenneberg T, Wirz-Justice A, Merrow M. Life between clocks: daily temporal patterns of human chronotypes. Journal of Biological Rhythms 2003;18:80-90.
Roenneberg T, Kuehnle T, Pramstaller PP, Ricken J, Havel M, Guth A, Merrow M. A marker for the end of adolescence. Current Biology 2004:14;R1038-1039.
Zavada A, Gordijn MC, Beersma DG, Daan S, Roenneberg T. Comparison of the Munich Chronotype Questionnaire with the Horne-Ostberg’s Morningness-Eveningness Score. Chronobiology International 2005;22:267-278.
Kantermann T, Juda M, Merrow M, Roenneberg T. The human clock’s seasonal adjustment is disrupted by daylight saving time. Current Biology 2007:17:1996-2000.
Rybak YE, McNeely HE, Mackenzie BE, Jain UR, Levitan RD. Seasonality and circadian preference in adult attention-deficit/hyperactivity disorder: clinical and neuropsychological correlates. Comprehensive Psychiatry 2007;48:562-71.
Archer SN1, Carpen JD, Gibson M, Lim GH, Johnston J.D, Skene DJ, von Schantz M. Polymorphism in the PER3 promoter associates with diurnal preference and delayed sleep phase disorder. Sleep 2010;33:695-701.
Pagani L, Semenova EA, Moriggi E, Revell VL, Hack LM, Lockley SW, Arendt J, Skene DJ, Meier F, Izakovic J, Wirz-Justice A, Cajochen C, Sergeeva OJ, Cheresiz SV, Danilenko KV, Eckert A, Brown SA. The physiological period length of the human circadian clock in vivo is directly proportional to period in human fibroblasts. PLoS One 2010;5:e13376.
Monk TH, Buysse DJ. Chronotype, bed timing and total sleep time in seniors. Chronobiology International 2014: e-pub abstract pending publication.