Interactive picture of circadian rhythm sleep disorders: an outer 24-hour schedule ring, an inner body-clock ring that turns late for DSPS or early for ASPD, a moved sleep window for shift work, and a bright light marker that turns the inner ring back
Circadian disorders are not broken clocks. They are good clocks set to the wrong hour
In DSPS and ASPD the internal rhythm still runs a tidy 24 hours; it is simply set later or earlier than the schedule the world demands. In shift work the clock is set fine and it is the schedule that moved. Shift the internal clock later or earlier and watch where sleep onset lands on the outer 24-hour ring, then add bright light at a time of day and watch which way the inner ring turns.
Body clock vs schedule
DSPS · clock +4 h
Choose a pattern, shift the internal clock, then add light at a time of day. Watch the inner ring and where the purple sleep onset lands against the amber window.
Sleep onset 03:00 · 4 h late for 23:00
Pattern
Internal clock
Internal clock shift+4 h
−6 h (ASPD)0+6 h (DSPS)
Bright light therapy
Sleep onset
03:00
external time
Gap to schedule
4 h
late for 23:00
Knowdrop
Know This
In DSPS and ASPD the rhythm itself is intact; what is wrong is its timing against the schedule, and bright light therapy moves that timing.