Domains for capture in epidemiological studies.
| Domain | Variable | Circadian impact |
| Working time | Work hours/week | |
| Night work (non-day shift work) | At least 3 h of work between midnight and 05:00 | Required to estimate phase shift and sleep perturbation |
| Duration | Years employed in non-day shift work | Duration of non-day shift work |
| Intensity | No of non-day shifts per month/year | Recovery time off between work periods |
| Cumulative exposure | Duration times intensity over the work history | Dose (burden) of non-day shift work |
| Permanent night shift (not rotating) | No of consecutive days of night work, followed by number of days off | Permanent night work is less disruptive only if phase shift is maintained also on days off |
| Rotating type | Continuous (365 days/year) or discontinuous (interruption on weekend) | Different rotating shift schedules have a different impact on phase shift and adjustment |
| Direction of rotation | Forward (morning→afternoon/evening→night) backward (afternoon/evening→morning→night) | Forward rotating shift schedules are less disruptive than backward ones |
| Rate of rotation | Daily change, 2–3–4 day change, weekly, fortnightly change, etc | Rate of rotating shift schedules (fewer nights in a row) may have different impact on circadian disruption |
| Morning shift | No of consecutive days of early morning shift (before 06:00) | The earlier the morning shift starts, the more disruptive it is |
| Start and end time of shifts | Defines displacement from solar day and duration of the working hours | May be relevant for phase shift, sleep deficit, and fatigue |
| Rest periods after shift | No of rest-days after night shifts | The shorter the rest period between shifts, the shorter the sleep and recovery |
| Jetlag | No of time zones crossed; eastward versus westward | Given the low prevalence in the general population, this is probably only needed in cohort studies of frequent trans-meridian travellers (eg, air crews), whereas jetlag studies should also include questions on shift work, since these often go hand in hand |
| Sleep | Sleep duration in relation to type of shift; naps; sleep quality; sleepiness; sleeping problems (circadian disruption); possibility to sleep on duty (night shifts) | Sleep/wake cycle and timing of sleep are important in phase shift and resetting, but they may also act as independent risk factors |
| Light at night | During sleep period, during night shift, at leisure time | Both timing and intensity are important on circadian phase shift |
| Characteristics of the individual | Diurnal type (morning person, evening person, neither) | It influences differently adjustment and tolerance to night and morning shifts |