Distal skin temperature stabilization is tightly coupled with sleep onset across structured and free-living...

We sought to characterize the time course and variability of distal skin temperature increase (DSTinc) around sleep onset with a wearable sleep tracker to evaluate its utility as a marker of sleep onset outside a sleep laboratory.

DST was measured using the Oura Ring over multiple nights and naps in 42 healthy adolescents living in a dormitory (16.4 ± 0.9 years; 224 nights, 77 naps; 52% female) and over multiple nights in 45 mostly healthy older adults living at home (75.0 ± 3.3 years; 197 nights; 52% female). N2 sleep onset (N2SO) was determine by EEG. Analyses focused on the period between time attempting to sleep (TATS) and N2SO. Relationships between DSTinc characteristics and N2SO latency, timing and temperature were explored in adolescents using linear mixed-effects models, controlling for age and sex. Analogous analyses were conducted in older adults to test generalizability to a different age group.

DSTinc started 9.2 ± 8.6 min before TATS in adolescents (naps: 8.0 ± 7.8 min) and ended 4.8 ± 8.0 min before N2SO (naps: 3.2 ± 4.6 min after N2SO). Temperature at the end of the nocturnal DSTinc was 35.4 ± 0.4°C (naps: 35.2 ± 1.3°C), with an increase of 4.7 ± 2.5°C (naps: 4.6 ± 2.2°C). Earlier DSTinc was associated with a shorter N2SO latency (ß_nights = 0.26, ß_naps = 0.33). Higher final DSTinc values were associated with higher N2SO temperatures (ß_nights = 0.57, ß_naps = 0.59). The same associations held in older adults, with similar patterns in DSTinc timing and level relative to N2SO.

The rise in peri-sleep DST shows a consistent temporal profile for both naps and nocturnal sleep; it is similar in different environments, and across age and sex.