Figure 3.

Illustration of the complexity of CRSWD. Melatonin data are plotted with respect to clock time; red circle indicates time of DLMO; dashed blue vertical line represents bedtime. (A) “Classic” DSWPD in which the timing of the circadian system and the timing of sleep are both occurring later than normal (circadian phase of DLMO ~02:00 am, bedtime ~04:00 am; see also Figure 2, B). (B) A case where bedtime is at a conventional hour (midnight) but DLMO is occurring later than normal (~02:00 am). Such a patient would likely experience prolonged sleep latency and may be misclassified as having sleep onset insomnia, rather than DSWPD. (C) A case where DLMO is occurring at a conventional hour (~10:00 pm) but bedtime is occurring much later than normal (~04:00 am). According to current standards, such a patient would not have had their DLMO assessed in the process of being diagnosed as having DSWPD. In this case, treatments designed to shift circadian phase earlier (morning light, evening melatonin) may be ineffective, given that circadian phase is already occurring at a conventional time, despite the fact that sleep is not occurring at an appropriate circadian phase (see also Figure 2, C).