Wrist actigraphy for determination of daytime midpoints is a potential tool to identify objective chronotype in persons with bipolar disorder (BD), which is associated with later sleep and daily activity (evening rather than morning chronotype), a recent study found. Given the ubiquity of accelerometer-based devices (eg, actigraphs/smartphones) worn/used during daytime and tendency to recharge rather than wear at night, researchers assessed chronotype using daytime (rather than sleep) interval midpoints. 61 participants with BD type I (BD-I) or II (BD-II) and 61 healthy controls completed 25–50 days of continuous actigraphy. The Composite Scale of Morningness (CSM) was completed by a subset of this group. They found:

• BD vs controls had delayed daytime midpoint and greater midpoint variability.
• Stratifying by gender and age, females and adolescents with BD had delayed and more variable daytime midpoints vs controls.
• Adults with BD had greater midpoint variability than controls.
• Within-person mean and standard deviations of daytime midpoints were highly correlated with sleep midpoints.
• Daytime midpoint mean was also significantly correlated with the CSM.