Table 1.

Definitions of different measures related to sleep-wake variability

Sleep variability measure	Operational definition	Measurement tool	Reference number for studies using measure
Standard deviation of sleep duration	Standard deviation of nightly sleep duration across the total number of days assessed	Wrist-worn actigraphy1	[12••,13•]
	Higher values indicate greater variability of night-to-night sleep duration
Standard deviation of sleep-onset timing	Standard deviation of nightly sleep onset timing across the total number of days assessed	Wrist-worn actigraphy1	[12••]
	Higher values indicate greater variability of night-to-night sleep onset timing
Sleep Regularity Index	The probability of the same sleep/wake state being observed at any two time points, 24-h apart (30-s epochs)	Wrist-worn actigraphy	[15•]
Interdaily Stability Index	Days are divided into 24 1-h bins; variance in activity explained by these bins is compared to the total variance across days	Wrist-worn actigraphy	[7•,8]
	Scores are between 0 and 1 with lower scores indicating less stability across days
Social jetlag	Difference in sleep patterns on work vs. non-workdays, measured by evaluating difference is sleep midpoint (midpoint between sleep onset and sleep offset) on weekdays vs. weekends	Wrist-worn actigraphy or self-reported wake and sleep times on weekdays and weekends	[7•,18–20]

¹SD of sleep parameters could also be calculated from multi-day self-reported sleep data, although not done in these studies