Why Do I Fall Asleep So Fast?

Some people can nod off quickly, sleep deeply, or nap anytime and anywhere. But being able to fall asleep so fast can actually be a symptom of a sleep disorder. 

Healthy people normally take about 10 to 20 minutes to fall asleep, with this time referred to as a sleep latency period. Dropping off right away might be because you're so tired but it also may be a sign of an underlying health issue like sleep apnea, or a thyroid issue (known to cause sleep latency changes).

This article explains the science of getting sleepy. It also discusses sleep disorders that could leave you feeling sleepy throughout the day.

How Does Sleepiness Occur?

First, it's important to understand how we become sleepy. The typical process involves the following:

• A chemical called adenosine builds up as you're awake, rising until levels create the homeostatic sleep drive, or the need for sleep.
• Your lymphatic system acts like a filter to clear the adenosine from your brain while sleeping. When you wake up in the morning, the levels of adenosine—and sleepiness—are at their lowest. If you've slept well, you feel refreshed. 

If you were awake for 30 straight hours, you would feel extremely sleepy. But even staying up past your normal bedtime can mean you fall asleep faster because your adenosine levels are up.

How Long Should It Take To Fall Asleep?

In healthy people, sleep latency (the time until you fall asleep) is about 10 to 20 minutes. If it takes longer than 20 to 30 minutes, it could be a sign of insomnia. But falling asleep in less than five minutes could signal an unhealthy level of sleepiness, too.

You're considered "asleep" when your muscle tone relaxes and the electrical waves in your brain slow down. These brain waves are called theta activity. Theta waves occur at a speed of four to eight times per second (hertz). By comparison, electrical waves in an awake, alert brain travel at twice this rate.

These sleep-related brain changes are measured by tracking the electrical activity of the brain. Sleep specialists use an electroencephalogram (EEG) as part of a sleep study, called a polysomnogram. Electrodes are placed on the scalp to measure brain waves and record when various stages of sleep occur, including tests for sleep latency.

Reasons for Falling Asleep Fast

There are a number of reasons for why you fall asleep fast, including the possibility that a short sleep latency period is normal for you. In most cases, though, there is a contributing factor or condition.

Sleep Deprivation

The most common cause of sleepiness is sleep deprivation. If you don't get enough hours of sleep to feel rested and to clear away the adenosine, you will fall asleep faster. Most people need about seven to nine hours of sleep. Some people may need more or less.

If you fall asleep quickly, take naps, doze accidentally, or sleep in on the weekends, you may be sleep deprived. A little extra sleep may be all it takes to ease your sleep debt.

Sleep Fragmentation

If you're not getting quality sleep or you wake up often during the night, this can also lead to falling asleep too quickly. Waking up a lot is called sleep fragmentation. Your sleep is literally broken up. A common cause is sleep apnea.

In people with sleep apnea, breathing stops briefly many times during the night. These breathing problems can wake you up. Sleep apnea is associated with other symptoms, including teeth grinding, snoring, and frequent trips to the bathroom at night. Fortunately, effective treatments exist to restore sleep quality.

Other Sleep Disorders

Other disorders can fragment sleep as well. One possibility is restless legs syndrome. It causes an uncomfortable feeling that you need to move your legs. Narcolepsy is another possibility. This sleep disorder causes you to slip into sleep without warning during waking hours.

Other causes of sleep fragmentation can include:

• Gastroesophageal reflux disease (GERD)
• Fibromyalgia and chronic pain
• Vitamin deficiencies that interfere with sleep
• Atherosclerosis (blocking of the arteries)

When sleep specialists can't pinpoint exactly why you're so sleepy, it may be diagnosed as idiopathic hypersomnia. That's the medical term for excessive sleepiness with no known cause.

Thyroid Disorders

Some studies have shown that people with hypothyroidism (low thyroid levels) have longer sleep latency periods, even when their thyroid symptoms are below levels considered clinical. The research on the connection has led to mixed results, however, with no clear understanding of the link between thyroid function and sleep.

The connection may be more established in people with hyperthyroidism (excessive thyroid function) who have trouble falling asleep. More than one study has shown a relationship, including one that showed a direct correlation between insomnia and thyroid hormone levels.

Hyperthyroidism also may contribute to other conditions that disrupt sleep, such as anxiety. People with hypothyroidism may be more at risk for sleep apnea and related disruptions.

Testing

The simplest way to measure sleepiness is by completing a questionnaire called the Epworth sleepiness scale. If you score higher than 10 on this scale, you probably have excessive sleepiness. The next step may be a formal sleep study.

The multiple sleep latency test (MSLT) can also be used to measure sleepiness. It is sometimes used to check for narcolepsy, and often follows a formal sleep study. With an MSLT, you are given a chance to take 20-minute naps in a sleep clinic, every two hours during a day. 

Sleep specialists record when your rapid eye movement (REM) sleep starts during a nap. If REM sleep starts within 15 minutes in two or more of your naps, you may be diagnosed with narcolepsy.

However, studies show a wide variation of results with an MSLT test; children, for example, don't appear to follow the same patterns as adults.

When to Talk to Your Healthcare Provider

If you find it too easy or too hard to fall asleep, or if you have trouble staying asleep, you may have a sleep disorder. Symptoms of daytime fatigue, with cognitive challenges like forgetting things or simply dozing off too easily, may be clues to problems with your sleep architecture.

They also may point to an underlying health condition, like a thyroid disorder that requires diagnosis and treatment. Your provider can assess the cause of your sleep latency challenges, sometimes with tests that include a formal sleep study, and then discuss further options with you.

In some cases, lifestyle changes (like establishing a bedtime routine or avoiding late-night screen time) may be a part of improving your overall sleep hygiene. You also may want to discuss using a wearable device, like a watch, or a pad tracker placed under the mattress to monitor your sleep.

Summary

Feeling sleepy is the result of the chemical adenosine. It builds up in your brain while you're awake. Sleep resets the adenosine levels.

If you're falling asleep fast, it could be because you're not getting enough quality sleep during the night. You may be sleep-deprived, which could explain the need for naps and the tendency to drift off even when you don't mean to.

Disorders such as sleep apnea, restless leg syndrome, narcolepsy, and other neurological conditions could be the cause. To find out exactly what the problem is, a sleep specialist could give you sleep-related questionnaires, an imaging test, or a formal sleep study.