Sleep disruption severity period messages describe the level of sleep disruptions detected during each period of the night.
messageIndexStringIndex of the message among the messages of the same kind in the file, starting at 0. Other messages use it to reference this one.
timestampDateTimeMoment the message refers to, in UTC. FIT stores it as seconds since the FIT epoch (December 31, 1989, 00:00:00 UTC); decoders usually expose it as a regular date.
severityEnumSeverity of the disruptions over the period: none, low, medium or high.
nonehighlowmediummesgNumStringGlobal message number identifying the kind of message. Decoders use it to know which profile to apply to the data.