Hd (od 21:00) -

The phrase most commonly appears in technical manuals for smartwatches and fitness trackers. It typically refers to the Sleep Monitoring (Sleep Test) window . Meaning and Function

: Many devices, such as those using the TimeTune or similar fitness apps, define the "HD" or health-monitoring period for sleep starting from 21:00 (9:00 PM) until the morning (usually 9:00 AM). HD (od 21:00)

Depending on where you saw this in a report, it could also refer to: The phrase most commonly appears in technical manuals

: Historical or local administrative reports referencing a "nightly curfew" (zákaz nočního vycházení) starting at 21:00. HD (od 21:00)

: A television program or sports match scheduled for a High Definition (HD) broadcast starting at 21:00 .