An individual’s sleep latency can be an indicator of the overall quality of their sleep. Quickly falling asleep during the day and rapidly entering REM sleep can suggest a condition such as narcolepsy and idiopathic hypersomnia. The Multiple Latency Test is often used to diagnose these conditions.
What Is Sleep Latency?
Sleep latency, or sleep onset latency, is the time it takes for you to go from being fully awake to sleeping. Sleep latency and the time it takes to reach rapid eye movement sleep (REM sleep) are important factors in the amount and quality of your sleep.
Getting enough sleep is very important for proper brain function, as well as the function of almost every organ. A lack of sleep or poor quality of sleep can affect the heart, lungs, and brain, as well as disturb the functioning of systems, such as metabolism and disease resistance. A chronic deficiency in either the quantity or quality of sleep can also increase the risk of numerous disorders, including depression, hypertension, heart disease, neurologic disease, diabetes, and cancer.
A multiple sleep latency test is often performed to indicate the amount and quality of sleep that you're getting and diagnose sleep disorders, such as narcolepsy and idiopathic hypersomnia.
When Is a Sleep Latency Test Done?
Your doctor will order a sleep latency test if you have symptoms of narcolepsy or idiopathic hypersomnia. The main symptom of these conditions is excessive daytime sleepiness, which is characterized by tiredness and a lack of energy during the day, even after a sufficient night’s sleep.
Narcolepsy and idiopathic hypersomnia are also associated with the following symptoms:
- Cataplexy, or a sudden weakening of your muscles, which is often caused by strong emotions
- Sleep paralysis
- Hallucinations that happen while you are dozing or falling asleep
- Waking up often during the night
What Is REM Sleep?
Rapid eye movement (REM) sleep is the stage of sleep when you dream the most. Two parts of the brain, the thalamus and amygdala, are active during REM sleep, creating images, sounds, and other sensations and allowing us to process emotions. REM sleep first occurs about 90 minutes after falling asleep and continues in 90 to 120 minutes cycles throughout the night.
People with narcolepsy enter the REM stage much more quickly after falling asleep. Narcolepsy can be diagnosed from a multiple sleep latency test based on the appearance of REM sleep during the 20-minute nap.
How Is the Multiple Sleep Latency Test Performed?
The Multiple Sleep Latency Test involves taking a series of 20-minute naps every two hours. The test usually consists of four to five naps. You'll be monitored to determine when you're awake, asleep, and in REM sleep. The time it takes you to fall asleep and reach REM sleep is measured. You'll be woken up after having slept for 15 minutes. If you don’t fall asleep, the nap trial will end after 20 minutes.
A polysomnogram is often done the day before to ensure that you have had an adequate night's sleep before the test.
How Are Results Interpreted?
The test results are considered positive if the average time required to fall asleep is below eight minutes. The normal delay in falling asleep in adults is between 10 and 20 minutes.
If your mean latency was below eight minutes and you experienced REM sleep during no more than one nap, you may have idiopathic insomnia, which causes excessive sleepiness during the daytime.
If your mean latency was below eight minutes and you experienced REM sleep during no more than two naps, you may have narcolepsy. People with narcolepsy fall asleep quickly and have impaired regulation of REM sleep. The impaired REM sleep regulation causes REM sleep during the day.
The Multiple Sleep Latency Test results are often combined with polysomnogram results before a diagnosis is made.
How Should You Prepare for a Sleep Test?
The Multiple Sleep Latency Test can be disrupted by several factors, such as:
- Caffeine use
- Medication or drug use, such as amphetamines or cocaine
- Lack of sleep
When preparing for a Multiple Sleep Latency Test, you may have to take certain measures to ensure the results are as accurate as possible, such as keeping a sleep diary during the two weeks before the test, maintaining a regular sleep schedule, reducing the use of stimulants like caffeine, sleeping sufficiently on the night before the test, and performing a drug test on the morning of the test to ensure that the results aren’t affected by any substances. Your doctor will advise you on the necessary measures.