Τhe human body produces its own melatonin starting two hours before bedtime, provided the lighting is dim. This natural action is known as “dim light melatonin onset” (DLMO) and helps keep the body on a regular sleep-wake schedule. DLMO is considered the best test available, a “gold standard”, for measuring Melatonin levels and Circadian Rhythm Disorders.
DLMO is useful for determining whether an individual is entrained (synchronized) to a 24-h light/dark cycle. DLMO is also useful for assessing phase delays or advances of rhythms in entrained individuals. DLMO marker is also useful for identifying optimal application times for therapies such as bright light or external melatonin treatment.
If an individual‘s circadian phase is normal, then his DLMO will be observed about 2 to 3 hours prior to habitual sleep. Normal DLMO time points are:
- Adults 07:30 - 10:00 pm
- Children (6-12 yrs.) 07:30 - 09:00 pm
An abnormal timing of DLMO indicates a circadian rhythm sleep disorder and provides a clue for the optimal timing of treatment with exogenous 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 the evening melatonin production measured in dim light, i.e., dim light melatonin onset, DLMO.
DLMO is believed to accurately represent the timing of the central circadian clock (suprachiasmatic nucleus, SCN), as the secretion of melatonin from the pineal gland is controlled by the SCN. Typically, melatonin levels begin to increase in the 2–3 h before the usual onset of nocturnal sleep, peak in the early morning hours, and decrease to daytime levels around usual waking.
The most noticeable feature of circadian rhythms is the sleep/wake cycle. But there are other circadian rhythms including swings in many hormones throughout the day, the body temperature cycle, appetite, and the times of best alertness. Ideally, these rhythms are in sync with each other and with the light-dark cycle in nature. Most humans are awake during daylight hours and sleep during darkness - it’s the ideal scenario for overall health.
DLMO is a convenient phase marker to measure, as it can be obtained noninvasively from saliva, and can require only a relatively short window of sampling of ~6–8 hours.
Circadian Rhythm Sleep Disorders
Circadian Rhythm Sleep Disorder (CRSD) is an abnormality of the body’s internal clock, in which a person is unable to fall asleep at a normal evening bedtime, although able to sleep reasonably well at other times dictated by their internal rhythm. They cannot fall asleep when desired, so they complain of insomnia. They have trouble waking up when desired, 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 the sleep itself.
One factor common to these disorders is inflexibility: even when physically tired or sleep deprived, sufferers cannot make up for lost sleep outside of their hard-wired sleep times. This factor is generally misunderstood by people who do not suffer from these disorders, leading to the conclusion that those who do are just lazy or haven’t tried hard enough to live on society’s schedule.
In addition to the sleep/wake cycle, the internal coordination of the various other rhythms may also be faulty. For example, some hormones may be on a different daily cycle than others, and this lack of coordination between systems may produce other symptoms in addition to the sleep disorder. This is believed to be the cause of the discomfort of jet lag.
The International Classification of Sleep Disorders Revised (ICSD-R) lists 6 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