Chronotype differences in circadian rhythms of temperature, melatonin, and sleepiness as measured in a modified constant routine protocol

Figures & data

Table 1 Mean sleep onset times, final wake-up times, and total sleep time for morning (MT) and evening type (ET) participants in the week prior to the laboratory session showing the difference and t-test results of the difference

	MT	ET	Difference hours	t
Sleep onset time	2324 (1.07)	0159 (1.28)	2.58	4.9*
Final wake-up time	0748 (1.0)	1020 (1.59)	2.53	4.29*
Total sleep time (hrs)	8.15 (0.64)	8.20 (0.73)	0.05	n.s.

Note:

* P < 0.001.

Figure 1 The mean times for the morning and evening type groups of their typical sleep period (horizontal shaded bars), of their subjective sleepiness minimum (SSmn) and maximum (SSmx), objective sleep propensity maximum (the maximum sleep in the 15-minute hourly sleep opportunities, SPmx and sleep propensity minimum (SPmin), dim light urinary melatonin onset (DLMO), and core temperature minimum phase, Tmin (down arows). The same phase markers for the two groups are connected with thin dashed lines so that the slopes of the connecting lines can indicate the relative differences between the two groups. It can be seen that the circadian and sleep timings are all about 2–3 hours later for the evening types. However, the subjective sleepiness phases show the greatest differences between groups. The morning types feel least sleepy in the early afternoon and the evening types least sleepy or most alert in the late evening, only a few hours before their typical bedtimes. The morning types then feel most sleepy before their Tmin (down arrow) but the evening types feel most sleepy after their Tmin at about their typical wake time.

Table 2 T-tests for the differences between the phase markers of morning type (MT) and evening type (ET) participants

Phase marker		MT clock time (SD)	ET clock time (SD)	Difference (hours)	t value
Core temperature	Tmin	0513 (1.62)	0728 (1.87)	2.25	2.88**
Melatonin	Max	0420 (0.93)	0620 (0.7)	2.00	2.55**
	DLMO	2148 (1.4)	2436 (0.84)	2.80	5.42***
Sleep propensity	Min	2128 (3.5)	2334 (2.88)	2.10	1.5
	Max	0759 (3.83)	1120 (4.74)	3.35	1.79*
Subjective sleepiness	Min	1318 (3.28)	2229 (3.58)	9.18	6.02***
	Max	0352 (1.45)	0922 (4.33)	5.50	4.11***

Notes:

* P < 0.05;

** P < 0.1;

*** P < 0.001.

Abbreviations: DLMO, dim light melatonin onset; SD, standard deviation.