Living organisms, including humans, have an internal, biological clock that helps them anticipate and adapt to the dramatically different phases of the day. This regular adaptation is referred to as the circadian rhythm 1 2 .
Circadian rhythm or the “body clock” is a cycle lasting on average a bit more than 24 hours. The clock regulates critical body functions such as hormone levels, sleep, body temperature, immune functions, blood pressure, behavior, and metabolism. 1 2
Located in a region of the brain called the suprachiasmatic nucleus, this clock is controlled by various environmental signals, most importantly the light–dark cycle and melatonin – a hormone in charge of sleep regulation and the circadian clock. Melatonin is produced in the pineal gland when the eyes send signals to the biological clock that it is dark. Its levels rise in the evening and remain elevated throughout the night, promoting sleep. Upon exposure to light in the morning, the circadian clock delays melatonin release 3 .
About 3% of the adult population suffer from a circadian rhythm sleep disorder (CRSD) 4 . 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 4 .
CRSD can be divided into 6 distinct types according to their root cause:
Delayed sleep phase syndrome (DSPS) – People with this syndrome need an extensive amount of time to fall asleep at a conventional time, and similarly have great difficulty waking up when they try to get up at a conventional morning time. DSPS is the most frequent type of Circadian Rhythm Sleep Disorders, and often first occurs in adolescence or young adulthood 5 6 .
Advanced Sleep Phase Type (ASPS) – People 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. ASPS is more likely to be observed in the elderly than in young adults 6 .
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 light 7 .
Jet Lag – Jet lag imbalances our body’s natural “biological clock” due to traveling between different time-zones. 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 eastward 8 . Jet lag symptoms last until the circadian system is realigned with the new time zone.
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 accidents 9 .
Our wellbeing is affected when there is a mismatch between our external environment and this internal biological clock. Chronic misalignment between our lifestyle and the rhythm dictated by our inner timekeeper is associated with increased risk for various diseases. 4 .