The definition is "the amount of ambient light/darkness to which a subject is exposed in a period of time"; also mapped to CDISC "the period of light that a subject is exposed to in a period of time, usually expressed as the amount of time in a 24 hour cycle".
Row 4
Can only be encoded if the light-dark cycles are aligned to a 24 hour clock. May be multiple if either multiple cycles occur during a 24 hour period, or if the cycle is longer than a 24 hour period and a multiple of 24 hours in duration.