Feb 24, 2008

Glossary of shiftwork terms: shiftwork, sleep, circadian rhythms, and more

Chronobiology: A subdiscipline of biology concerned with the timing of biological events, especially repetitive or cyclical phenomena, in individual organisms.

Circadian: A term derived from the Latin phrase “circa diem,” meaning “about a day”; refers to biological variations or rhythms with a cycle of approximately 24 hours. Circadian rhythms are self-sustaining (i.e., free running), meaning that they will persist when the organism is placed in an environment devoid of time cues, such as constant light or constant darkness. For comparison, see diurnal, infradian, and ultradian.

Day shift: In an 8-hour shift operation, the day shift may also be called the first shift and typically begins between 6 am and 8 am. In a 12-hour shift operation, this shift typically also begins between 6 am and 8 am.

Diurnal: Varying with time of day. Diurnal rhythms may persist when the organism is placed in an environment devoid of time cues, such as constant light or constant darkness. Therefore, diurnal variations can be either light driven or clock driven. For comparison, see circadian.

Entrainment: The process of synchronization of a timekeeping mechanism to the environment, such as to a light-dark cycle. For comparison, see free running.

Evening shift: This most commonly refers to the second shift in an 8-hour operation and typically begins between 2 pm and 4 pm. Also often referred to as the swing shift.

Fatigue countermeasures: Any of an array of steps taken to help people maintain alertness, specifically to help shiftworkers maintain health and safety. These may include such things as napping and sleep strategies, managing the use of caffeine and other substances to promote alertness or sleep, lighting and temperature of the work environment, and many other areas.

Fixed schedule: A shift schedule in which workers are permanently assigned to a specific shift -- day, evening, or night. This contrasts with a rotating schedule.

Free running: The state of an organism (or rhythm) in the absence of any entraining stimuli. Typically, subjects are kept in constant dim light or constant darkness to assess their free-running rhythms. For comparison, see entrainment.

Graveyard shift: See night shift.

Infradian: A term derived from the Latin phrase “infra diem,” meaning “less than a day”; refers to biological cycles that last more than 1 day and, therefore, have a frequency of less than one per day. For comparison, see circadian and ultradian.

Masking: The obscuring of the “true” state of a rhythm by conditions that prevent its usual expression. Usually, the phase of an entrained rhythm or the absence of entrainment (e.g., in an animal that is unable to entrain because of some defect) is said to be masked by a light cycle. For example, the aversion of a nocturnal rodent to bright light results in its activity onset appearing to coincide with the absence of light, or “lights off,” when the animal actually has been awake for hours. For comparison, see entrainment.

Night shift: In an 8-hour shift operation, the night shift may also be called the third shift and typically begins between 10 pm and midnight. In a 12-hour shift operation, this shift typically begins between 6 pm and 8 pm. Also sometimes referred to as the graveyard shift.

Nonrapid eye movement (NREM) sleep: Sleep stages that include the “deeper” stages of sleep in which dreaming typically does not occur. Also referred to as slow-wave sleep. There are four non-rapid eye movement (NREM) sleeps stages. Stage 1 is a transitional phase, with brain activity slowing. This is the drowsy state when sudden movements and hallucinations may occur. In stage 2 sleep, muscular activity decreases and humans lose awareness of their environment. We stay in stage 2 sleep for approximately half of a typical session of sleep.
Stage 3 is a deep phase of sleep, characterized by slow-wave wave activity. This is the stage in which people experience sleep walking, sleep talking, and bed wetting. Stage 4 sleep is a deeper stage of sleep but similar to stage 3. For comparison, see rapid eye movement sleep.

Phase shift: A change in the phase of a rhythm. This change can be measured by observing a change in the timing of a phase reference point (e.g., activity onset or the nocturnal rise in the release of the hormone melatonin) from the timing expected based on previous, free-running cycles. Phase shifts may be either advances (i.e., the phase reference point occurs earlier than normal) or delays (i.e., the phase reference point occurs later than normal).

Phase-response curve (PRC): A graphical summary of the phase shifts produced by a particular manipulation, such as a light pulse or a pharmacological treatment, as a function of the phase (i.e., circadian time) at which the manipulation occurs. Defining the PRC to light has enabled researchers to understand and predict how entrainment to light cycles is accomplished.

Rapid eye movement (REM) sleep: A stage of light sleep characterized by rapid eye movements and associated with dreaming. REM sleep is the stage when humans dream most and muscle activity falls. Also called paradoxical sleep. For comparison, see nonrapid eye movement sleep.

Rota: See shift schedule.

Rotating schedule: A shiftwork schedule in which workers rotate between day and night shifts on a 12-hour shift schedule, or between day, evening, and night shifts on an 8-hour shift schedule. There are many factors to consider when setting the speed and direction of the rotation. This contrasts with a fixed schedule.

Shift schedule: The schedule of shifts to be worked. Also referred to as a rota, particularly in European countries. See also rotating schedule and fixed schedule.

Sleep stages: Human sleep consists of 5 stages, each with its own unique characteristics, through which humans cycle while they sleep. See rapid eye movement sleep and non-rapid eye movement sleep.

Suprachiasmatic nucleus or nuclei (SCN): A cluster of nerve cells located in the brain region called the hypothalamus that is responsible for generating and coordinating circadian rhythmicity in mammals.

Swing shift: See evening shift.

Ultradian: A term derived from the Latin phrase “ultra diem,” meaning “more than a day”; refers to biological cycles that last less than 1 day and, therefore, have a frequency of more than one per day. For comparison, see circadian and infradian.

Zeitgeber: A German word literally meaning “time-giver.” A time cue capable of entraining circadian rhythms. Light represents the most important Zeitgeber.



Sources: National Shiftwork Information Center research staff, http://www.niaaa.nih.gov/publications/arh25-2/85-93.htm


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