What sleep efficiency measures: TST divided by TIB
Sleep efficiency (SE) is calculated as total sleep time (TST) — the time actually spent asleep — divided by time in bed (TIB) — the total time spent lying in bed, whether asleep or awake — multiplied by 100 to give a percentage. For example, someone who is in bed for 8 hours and asleep for 6 hours 48 minutes has a sleep efficiency of (6.8 ÷ 8) × 100 = 85%.
This ratio is one of the core metrics recorded in a sleep diary, alongside total sleep time and sleep latency (how long it takes to fall asleep), and is a standard outcome measure in sleep medicine research and clinical practice. Because it depends on both how long someone sleeps and how long they spend in bed, two people with the same total sleep time can have very different sleep efficiency figures if one spends considerably longer lying awake in bed.
The ≥85% clinical convention
A sleep efficiency of 85% or higher is a widely used clinical convention for a normal result in sleep-diary assessment and cognitive behavioral therapy for insomnia (CBT-I) protocols. Efficiency between 75% and 84% is generally treated as mildly reduced, and below 75% as low — categories reflecting an increasing share of time in bed spent awake rather than asleep. These are conventions used in clinical practice and research, not a single universally fixed cutoff mandated by one governing body.
It is worth being precise about what this threshold does and does not represent: it is a benchmark for a diary- or wearable-derived efficiency ratio, not itself a diagnostic criterion for insomnia. A formal insomnia diagnosis involves broader clinical criteria — including how the sleep difficulty affects daytime functioning and how long the pattern has persisted — assessed by a clinician, not a single efficiency percentage from one or a few nights.
Sleep restriction as a component of CBT-I
Sleep restriction therapy is a structured technique within CBT-I, first described by Spielman, Saskin and Thorpy in 1987, in which time in bed is temporarily limited to more closely match a person's actual average sleep time. The rationale is that spending a large amount of time in bed awake — a pattern common in insomnia — dilutes sleep efficiency; by temporarily shortening the sleep opportunity window and then gradually expanding it as efficiency improves, treatment aims to consolidate sleep and raise sleep efficiency back toward or above the 85% clinical convention.
This technique is described here for educational purposes and is administered clinically as part of a structured CBT-I program, typically delivered or supervised by a trained clinician such as a sleep psychologist, because restricting time in bed can increase short-term daytime sleepiness and needs to be titrated carefully. The American Academy of Sleep Medicine's 2021 clinical practice guideline identifies behavioral and psychological treatments, including CBT-I components such as sleep restriction, as an evidence-based approach for chronic insomnia disorder in adults.
When low sleep efficiency signals something to discuss with a doctor
A single night of low sleep efficiency is common and not by itself concerning — an unusual schedule, stress, noise, or an off night can easily produce one low reading, and clinicians and sleep-diary protocols typically evaluate a week or more of entries rather than a single night. A persistent pattern of sleep efficiency well below 85%, especially alongside difficulty falling or staying asleep and daytime impact such as fatigue or difficulty concentrating, is a reasonable reason to bring the pattern to a healthcare professional's attention.
Low sleep efficiency on its own does not indicate a specific cause. It can result from many factors, ranging from an inconsistent sleep schedule to clinical insomnia or another sleep disorder. Only a clinician, considering a full history and, where appropriate, further assessment, can determine whether a persistent efficiency pattern reflects a condition requiring treatment.
Tracking sleep efficiency over time
Because night-to-night variation is normal even for good sleepers, sleep-diary protocols typically average a week or more of nightly entries rather than drawing conclusions from a single measurement. Recording time getting into bed, time getting up, and an estimate (or tracker-derived measurement) of total sleep time each night allows a running sleep efficiency figure to be calculated and trends to be observed over time, which is more informative than any single night's result.
よくある質問
What is sleep efficiency and how is it calculated?
Sleep efficiency is the percentage of time in bed actually spent asleep, calculated as total sleep time (TST) divided by time in bed (TIB), multiplied by 100. For example, 6 hours 48 minutes of sleep during 8 hours in bed gives a sleep efficiency of (6.8 ÷ 8) × 100 = 85%. It is a standard outcome measure recorded in sleep diaries and used in sleep medicine research and clinical practice.
What counts as a good sleep efficiency score?
A sleep efficiency of 85% or higher is the widely used clinical convention for a normal result, applied in CBT-I and sleep-diary assessment. Efficiency between 75% and 84% is generally considered mildly reduced, and below 75% is considered low. These are conventions used in clinical practice, not a strict pass/fail diagnostic threshold on their own.
What is sleep restriction therapy?
Sleep restriction therapy, first described by Spielman, Saskin and Thorpy in 1987, is a component of cognitive behavioral therapy for insomnia (CBT-I) in which time in bed is temporarily limited to more closely match a person's actual average sleep time, then gradually expanded as sleep efficiency improves. It is intended to consolidate fragmented sleep and is typically administered or supervised by a trained clinician as part of a structured treatment program, because restricting time in bed can temporarily increase daytime sleepiness.
Does low sleep efficiency always mean insomnia?
No. Low sleep efficiency on a given night or short period can result from many factors, including stress, an unusual schedule, or a single off night, and does not by itself confirm insomnia. A formal insomnia diagnosis involves broader clinical criteria assessed by a healthcare professional, including how often and for how long a sleep-difficulty pattern persists and its effect on daytime functioning, not a sleep-efficiency percentage alone.
How many nights of data do I need before sleep efficiency is meaningful?
Sleep-diary protocols in clinical and research settings typically evaluate a week or more of nightly entries rather than a single night, because night-to-night variation in sleep efficiency is normal even among people who sleep well. A persistent pattern across multiple nights is far more informative than one night's result, whether that result happens to look unusually good or unusually poor.
参考文献
- Spielman AJ, Saskin P, Thorpy MJ. Treatment of chronic insomnia by restriction of time in bed. Sleep 1987; 10(1): 45–56.
- Buysse DJ, Ancoli-Israel S, Edinger JD, Lichstein KL, Morin CM. Recommendations for a standard research assessment of insomnia. Sleep 2006; 29(9): 1155–1173.
- Edinger JD, Arnedt JT, Bertisch SM, et al. Behavioral and psychological treatments for chronic insomnia disorder in adults: an American Academy of Sleep Medicine clinical practice guideline. Journal of Clinical Sleep Medicine 2021; 17(2): 255–262.