Sleep in myalgic encephalomyelitis/chronic fatigue syndrome shows marked night-to-night fluctuation under free-living conditions-results from a matched case-control study.
Saurel M, Fornasieri I, et al. • Journal of clinical sleep medicine : JCSM : official publication of the American Academy of Sleep Medicine • 2026
Under free-living conditions, sleep in ME/CFS is characterized not only by impaired sleep efficiency but also by pronounced night-to-night variability, despite relatively stable bedtime compared to controls.
Key Findings
Results
People with ME/CFS spent significantly more time in bed than matched controls but did not have significantly longer total sleep time.
Study used 7-day wrist accelerometry under free-living conditions in 38 ME/CFS participants and 38 matched controls.
Despite greater time in bed, total sleep time did not differ significantly between groups.
All comparisons of time in bed and sleep efficiency were reported at p < 0.05.
Sleep diaries and the Pittsburgh Sleep Quality Index (PSQI) were used concurrently with accelerometry.
Sleep efficiency (SE) was significantly lower in the ME/CFS group (p < 0.05).
The finding was consistent across both actigraphy and sleep diary measures.
38 ME/CFS participants were compared to 38 matched controls over 7 consecutive days.
Poorer sleep efficiency despite longer time in bed indicates greater time spent awake while in bed.
Results
ME/CFS participants showed significantly greater intraindividual variability (IIV) in sleep efficiency compared to controls.
Sleep IIV was quantified using three methods: the coefficient of variation, the root mean square of successive differences, and a Bayesian variability model.
Greater night-to-night fluctuation in sleep efficiency was a distinguishing feature of ME/CFS under real-life conditions.
The authors describe this as 'pronounced night-to-night variability' in sleep efficiency.
This finding was identified across the week-long monitoring period under free-living conditions.
Results
Sleep timing (bedtime) was more regular among ME/CFS participants compared to controls.
Despite greater variability in sleep efficiency, ME/CFS participants showed more stable bedtimes than controls.
This contrast between stable sleep timing and unstable sleep efficiency was a notable pattern in ME/CFS.
The finding suggests that behavioral regularity in sleep scheduling does not prevent night-to-night fluctuations in sleep quality.
Intraindividual variability metrics were applied to bedtime as well as sleep efficiency.
Results
Exploratory analyses did not detect clear differences in mean sleep variables or variability indices across ME/CFS severity subgroups.
ME/CFS participants were stratified by symptom severity using the Bell Disability Scale.
No clear differences were found between severity subgroups for either average sleep measures or night-to-night variability indices.
These analyses were described as exploratory in nature.
The lack of severity-based differences may reflect limited statistical power within subgroups or genuine homogeneity of sleep disturbance across severity levels.
Discussion
The study identifies sleep efficiency variability as a clinically relevant feature of ME/CFS that is not captured by single- or two-night laboratory assessments.
The authors note that 'most evidence relies on single- or two-night laboratory assessments that may not reflect habitual sleep under real-life conditions.'
Week-long wrist accelerometry was used to capture habitual sleep patterns.
The authors conclude there is 'a need for multi-night assessment and targeted strategies addressing sleep variability.'
Sleep IIV is described as 'a potentially critical dimension of sleep health' not previously systematically examined in ME/CFS.
Methods
This was a matched case-control study using concurrent objective and subjective sleep measures over 7 days.
38 people with ME/CFS were matched with 38 controls.
Participants wore a wrist accelerometer continuously for 7 days.
Concurrent measures included sleep diaries, the Pittsburgh Sleep Quality Index (PSQI), and the Epworth Sleepiness Scale (ESS).
Three distinct statistical methods were used to quantify intraindividual variability: coefficient of variation, root mean square of successive differences, and a Bayesian variability model.
What This Means
This research suggests that people with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) not only sleep poorly on average, but also experience dramatic swings in sleep quality from one night to the next. The study had 38 people with ME/CFS and 38 healthy matched controls wear wrist activity monitors continuously for one week in their normal home environments, while also keeping sleep diaries and completing standard questionnaires. The results showed that people with ME/CFS spent more time in bed but did not sleep longer overall, had worse sleep efficiency (meaning more of their time in bed was spent awake), and had much greater unpredictability in how well they slept each night. Interestingly, despite this variable sleep quality, people with ME/CFS actually kept more consistent bedtimes than the healthy controls.
The finding about night-to-night variability is particularly notable because most previous research on sleep in ME/CFS has relied on one or two nights recorded in a sleep laboratory, which would miss this fluctuation entirely. This research suggests that the instability of sleep — not just its average quality — may be an important part of what makes sleep so unrefreshing for people with ME/CFS. The study also found no clear pattern linking symptom severity to sleep characteristics, though those analyses were exploratory.
From a practical standpoint, this research suggests that assessing sleep over multiple nights in real-world settings gives a more complete picture of sleep problems in ME/CFS than brief laboratory studies. It also points toward the possibility that treatments for ME/CFS should consider not just improving average sleep quality but also stabilizing it from night to night, though further research would be needed to develop and test such approaches.
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Saurel M, Fornasieri I, Del Sordo G, Chatain C, Fantini M, Gruet M, et al.. (2026). Sleep in myalgic encephalomyelitis/chronic fatigue syndrome shows marked night-to-night fluctuation under free-living conditions-results from a matched case-control study.. Journal of clinical sleep medicine : JCSM : official publication of the American Academy of Sleep Medicine. https://doi.org/10.1007/s44470-026-00079-7