Circadian Rhythm Circadian rhythm or the “body clock” is a cycle lasting on average a bit more than 24 hours. Circadian rhythms are self-sustained but also regulated by external factors, especially by daylight. They help organisms, including humans, to adapt to the environment’s schedule, and adjust their body and behaviour accordingly1.
Circadian Rhythm Sleep Disorders (CRSD) About 3% of the adult population suffer from a circadian rhythm sleep disorder (CRSD)2. CRSD is a disruption of the internal body clock, which is characterized by an abnormality in length and timing of the sleep-wake cycle in regards to the day-night cycle. CRSD can be caused by many factors such as shift work, blindness, jet leg etc. CRSD main symptoms are reduced alertness, nighttime insomnia, loss of appetite, depressed mood, poor coordination and reduced cognitive skills2.
CRSD can be divided into 6 distinct types according to their root cause (internal and external factors).
Internal (endogenous) factors:
- Delayed sleep phase syndrome (DSPS) – Individuals with this syndrome require an extensive amount of time to fall asleep at a conventional time, and similarly have great difficulty to wake up when they try to get up at a conventional morning time. DSPS is the most frequent type of Circadian Rhythm Sleep Disorders seen by the clinician, and often first occurs in adolescence or young adulthood3 4.
- Advanced Sleep Phase Type (ASPS) – Individuals with ASPS experience problems opposite to those of DSPS individuals. They have regular sleep-wake patterns but with an early sleep onset and offset times, much earlier than desired. Their sleep duration is normal and accompanied with a difficulty to stay awake in the evening for social events or to spend time with family. ASPS is more likely to be observed in the elderly than in young adults4.
- Irregular sleep wake phase type – This disorder is characterized by a normal 24-hour sleep duration, but with a disruption and fragmentation of the sleep-wake cycle- unrecognizable pattern and disorganized sleep and wake times, with insomnia and/or excessive daytime drowsiness.
- Free running type/ Non-24-Hour Sleep-Wake Disorder – A neurological sleep disorder in which a person’s sleep-wake cycle is longer than 24 hours. The person is unable to adjust his sleep-wake cycle to match the length of the day, therefore his sleep time progresses around the clock. This syndrome is common in totally blind people who are unable to detect light5. As most individuals live in a 24-hour world, a non-24 rhythm can lead to major difficulty in functioning in social environments, especially at work.
External (exogenous) factors:
- Jet Lag – Jet lag imbalances our body’s natural “biological clock” due to traveling between different time-zones. When traveling to a new time zone, the biological clock slowly adjusts to the new time zone, shifting from its original biological schedule to the new schedule. The severity of jet lag symptoms is affected by both the number of time zones crossed and the direction of travel, where flying westward is easier to tolerate than is flying eastward6. The greater the difference from home time zone, the more likely there will be jet lag, and the longer it will take to fully adjust.
- Shift-work sleep disorder (SWSD) – This disorder is generated in people whose shift work extends beyond the typical “nine-to-five” workday, wherein schedules are often early morning shift or night shifts. Rotating shifts, with day shifts on some days and night shift on others, disrupts the circadian system of the worker, leading to drowsiness during night work and difficulties falling asleep during the day. Night shift workers have shorter sleep duration on average than day or evening shift workers because they attempt to sleep at an inappropriate biological time. They usually go to sleep following work and thus begin each night shift after insufficient sleep and many hours of wakefulness, increasing the risk of injuries at work and car accidents7.
