What is ISL (Intermediate Sleep Stage) in terms of polysomnography?

Initial Sleep Latency (ISL) in Polysomnography

In polysomnography, ISL (Initial Sleep Latency) refers to the time it takes for a person to fall asleep after lights out, measured in minutes from the start of the recording to the first epoch of sleep. 1

Definition and Measurement

• ISL is a critical parameter in sleep studies that quantifies the transition time from wakefulness to sleep onset 1
• It is typically measured from lights out (beginning of the intended sleep period) to the first epoch scored as sleep (usually N1 sleep stage) 2
• ISL is considered a CRITICAL outcome measure for evaluating sleep disorders, particularly circadian rhythm sleep-wake disorders 1

Clinical Significance

• Prolonged ISL (>30 minutes) may indicate insomnia or delayed sleep phase disorder 2
• Shortened ISL (<5 minutes) may suggest sleep deprivation or disorders of excessive sleepiness 2
• ISL is used as a key diagnostic parameter in various sleep disorders, including:
  • Advanced Sleep-Wake Phase Disorder (ASWPD) 1
  • Delayed Sleep-Wake Phase Disorder (DSWPD) 1
  • Other circadian rhythm sleep-wake disorders 1

ISL Measurement Methods

• Polysomnography (PSG): The gold standard for objective measurement of ISL 3, 4
• Actigraphy: Provides an estimate of ISL through movement patterns 1
• Subjective reports: Patient-reported time to fall asleep (less accurate but clinically relevant) 1

ISL in Treatment Evaluation

• ISL is used to evaluate the effectiveness of various sleep disorder treatments:
  • Light therapy: In ASWPD, light therapy showed variable effects on ISL, with actigraphy showing a 1.85 minute increase and subjective reports showing a 41.02 minute increase 1
  • Melatonin treatment: In adults with DSWPD and depression, melatonin reduced ISL by 43.52 minutes as measured by PSG 1
  • In children with DSWPD, melatonin at 0.15 mg/kg reduced ISL by 43.80 minutes as measured by actigraphy 1

Interpretation Considerations

• ISL values must be interpreted in the context of the measurement method (PSG vs. actigraphy vs. subjective) 1
• The American Academy of Sleep Medicine has established standardized criteria for interpreting polysomnography data, including ISL 2
• Inter-rater reliability for sleep staging affects ISL measurement, with substantial overall agreement (Cohen's kappa 0.76) 5

Common Pitfalls in ISL Interpretation

• Different definitions and scoring criteria can result in significant variations in ISL values 2
• First-night effect may artificially prolong ISL due to unfamiliar sleep environment 4
• Medications (especially hypnotics, stimulants, or melatonin) can significantly alter ISL 6, 7
• Technical factors such as sensor placement and recording quality can affect accurate ISL determination 8, 4

Clinical Applications

• In treatment studies, ISL is used as an outcome measure to determine efficacy:
  • For DSWPD in adults without depression, melatonin treatment decreased ISL by 37.70 minutes 1
  • For children with DSWPD and psychiatric comorbidities, melatonin reduced ISL by 18.70 minutes 1
• ISL can help differentiate between various sleep disorders when considered alongside other sleep parameters 9

Remember that ISL is just one component of a comprehensive sleep evaluation and should be interpreted alongside other polysomnographic parameters and clinical information 2, 4.