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What Is Sleep Latency and Why It Matters

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What is it

Sleep latency is the term given to describe how long it takes to fall asleep. Sleep latency can be highly variable due to behaviors prior to bedtime, such as alcohol, medications, exercise, diet, and blue light exposure, among others. However, tracking sleep latency can provide additional data that can reflect changes in health, behavior, and their interaction.

How is it measured

Sleep latency is measured in minutes from the time an individual attempts to fall asleep to the time when the individual enters sleep. While this can seem rather easy to qualitatively assess for an individual, tracking changes in physiological metrics through photoplethysmography (PPG) and accelerometry provides improved insight as individuals may have difficulty reporting the time of initial sleep onset. By tracking metrics such as heart rate, heart rate variability, respiration rate, and limb movements, a good understanding of bed time and onset of sleep can be made.

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Correlations to health conditions

The connection between sleep onset latency and clinical outcomes is less clear than that of total sleep time and deep sleep duration, however, there appears to be correlations between them. It is important to note that directionality and magnitude of latency may or may not have clinical relevance based on the situation that an individual is in. For example, long sleep latencies can be indicative of disorders, particularly related to stress or insomnia; however, shortening sleep latency may not reflect positive changes, as sleep latency is decreased by high amounts of sleep debt and may not reflect an overall positive change. Further, substances such as alcohol may reduce sleep latency but may lead to lesser quality of sleep.

Many of the correlations between latency and health are drawn in anxiety and depression. These psychological disorders are fairly common and are known to affect sleep and sleep latency. However, sleep latency is associated with total sleep duration reduction, which has a feed-forward effect on sleep, where less sleep causes more anxiety and depression. Thus, it can be important to monitor sleep latency changes to catch trends before they become problematic.

Normal or acceptable ranges

The National Sleep Foundation acknowledges up to 30 minutes of sleep latency, regardless of age, as appropriate. Sleep latency of 31-45 minutes is listed as ‘uncertain’, which could be due to individual trends. It stands to reason that very short sleep latency (<5 minutes) could indicate problems with fatigue and sleep deprivation, however, the research on normative values in this range are unclear.

Interpreting trends

Although the clinical recommendations remain unclear, tracking sleep latency could be good for most individuals. This metric, inversely associated with total sleep duration, could provide insight into behavioral changes and how they affect sleep architecture. Should sleep latency trend negatively for an individual, behavioral interventions could be suggested to correct sleep latency, and potentially increase total sleep duration.