Circadian delay

Feeling tired need not mean the body is ready for sleep

Research-backed

Two physiological processes jointly regulate sleep. Sleep pressure builds the longer someone stays awake. Meanwhile, the body clock sends signals that sustain alertness on its own schedule, counteracting sleep pressure. A tiring day therefore need not mean the body has entered a window when sleep comes easily(Auger et al., 2015).

Group averages cannot replace an individual’s rhythm measurement

Research-backed

A systematic review found that people with attention-deficit/hyperactivity disorder (ADHD) tended toward later sleep schedules and later physiological rhythm markers(Coogan & McGowan, 2017).

Bedtime and internal rhythms provide different information. Sleep guidelines consider records of sleep schedules over several days separately from biological rhythm measurements. Late bedtimes alone cannot substitute for rhythm measurements(Auger et al., 2015).

An earlier body-clock marker need not mean earlier sleep

Research-backed

A randomized trial in adults with ADHD and delayed sleep phase used the time when saliva Melatonin began rising in dim light as a body-clock marker. This marker shifted earlier in the treatment groups. An exploratory follow-up analysis of the same trial, however, detected no shift to earlier sleep or overall sleep improvement(van et al., 2020; van et al., 2022).

Assessing whether an adjustment has worked requires considering internal rhythms and actual sleep separately. A change in one marker cannot be equated with an earlier sleep time.