The human body produces melatonin two hours before bedtime, provided that the lighting is dim. This natural action is known as “dim light onset melatonin secretion” (DLMO) and helps the body maintain a regular sleep-wake schedule.
The determination of DLMO is considered the best laboratory test available for measuring melatonin levels and circadian rhythm disorders.
The determination of DLMO is performed to check whether an individual is synchronized to the 24-hour light/dark cycle. The test is also helpful for assessing the delay or advance of the circadian rhythm phase in the examined individuals and determining the optimal times for applying treatments such as bright light or melatonin administration.
If a person's circadian rhythm is normal, then DLMO will occur approximately 2 to 3 hours before their usual bedtime. Standard DLMO timings are:
- Adults 07:30 - 10:00 PM
- Children (6-12 years) 07:30 - 09:00 PM
An abnormal DLMO timing indicates a disruption in the circadian sleep/wake rhythm and provides the necessary clues for the optimal timing of treatment with melatonin or bright light.
Measuring the Circadian Clock
The most reliable measure of the timing of the central circadian clock in humans is the onset of evening melatonin production measured in dim light, i.e., the onset of melatonin secretion in dim light, DLMO.
The determination of DLMO is believed to accurately represent the timing of the central circadian clock (suprachiasmatic nucleus), as the suprachiasmatic nucleus controls melatonin secretion from the pineal gland. Typically, melatonin levels increase 2–3 hours before nocturnal sleep onset, peak in the early morning hours, and decline to daytime levels around the time of usual awakening.
The most notable feature of circadian rhythms is the sleep/wake cycle. But there are other circadian rhythms, including the daily fluctuations of many hormones, the body temperature cycle, appetite, and times of best wakefulness. Ideally, these rhythms are in sync with each other and the light-dark cycle. Most people are awake during the day and sleep in the dark, which is the ideal scenario for optimal health.
DLMO is a convenient biomarker to measure. It can be obtained noninvasively from saliva and may only require a relatively short sampling period of 6-8 hours.
Circadian Rhythm Sleep Disorders
Circadian rhythm disorder is an abnormality of the body's internal clock in which a person cannot fall asleep at a regular bedtime. However, they can sleep quite well at other times, which is dictated by their internal rhythm. These people cannot fall asleep when they want to, so they complain of insomnia. In addition, they have difficulty waking up when they want to, so they complain of excessive sleepiness. It takes some insight to make the connection that the problem may be with the timing of sleep rather than sleep itself.
A common factor in these disorders is the presence of inflexibility: even when physically tired or sleep-deprived, sufferers cannot make up for lost sleep outside of their designated bedtimes. This factor is generally misunderstood by people who do not suffer from these disorders, leading to the conclusion that those who suffer are simply lazy or do not try hard enough to live according to the schedule of the rest of society.
In addition to the sleep/wake cycle, the internal coordination of various other rhythms can also be problematic. For example, some hormones may be on a different daily rhythm than others, and this lack of coordination between systems can cause other symptoms besides sleep disturbance. This is believed to be the cause of the discomfort of jet lag.
CRSD Subtypes
The International Classification of Sleep Disorders Revised (ICSD-R) lists six subtypes of circadian rhythm sleep disorder:
- Delayed Sleep-Phase Syndrome
- Non-24-Hour Sleep-Wake Disorder
- Advanced Sleep-Phase Syndrome
- Irregular Sleep-Wake Pattern
- Shift Work Sleep Disorder
- Jet Lag Syndrome
