Why self-report is what large cohorts actually use
Large cohort studies that follow thousands of people over years need sleep data that is cheap, repeatable, and easy to collect at scale. Self-reported sleep duration fits that need, and it is the measure behind most of the evidence connecting sleep to blood sugar. A multiethnic cohort of about 900 people used self-reported sleep duration to examine who developed type 2 diabetes, while measuring insulin sensitivity and secretion objectively [4]. A large cohort of postmenopausal women likewise used self-reported sleep characteristics to study the risk of treated diabetes [3].
That pattern matters for the question. When a cohort reports that short or long sleep predicts diabetes, the sleep measure is usually a questionnaire answer, not a night in a lab. So the practical answer for a researcher planning a large study is that self-report is the measure with a track record against blood sugar outcomes, even though it is not a perfect record.
What self-report gets wrong, and why it may still work
Self-reported sleep duration can overestimate or underestimate actual sleep, a limitation the postmenopausal cohort itself notes [3]. People are imperfect judges of how long they slept, and the error is not random. That is a real weakness if the goal is to measure sleep precisely.
But predicting blood sugar may not require precision. A study of over 1,000 Danish adults found that low self-reported sleep sufficiency was linked to poorer mental and physical health regardless of accelerometer-measured sleep duration, and the authors suggest self-reported sufficiency may be more closely correlated with health than measured duration alone [1]. That study did not look at blood sugar, so it cannot settle the question directly. It does, however, support the idea that how people feel about their sleep carries information that a device's duration count may not capture.
Where objective sleep measurement fits
Objective measurement has appeared in blood-sugar research, but not as a head-to-head challenger to self-report. One community-based cohort combined actigraphy-assessed sleep with self-reported sleep measures and examined inflammatory markers and insulin, linking poor sleep to raised blood sugar and insulin resistance [2]. Because the study used both kinds of measure together, it does not tell us which one predicted the outcome better.
A cross-sectional study of 424 rural older adults in northern Thailand linked self-reported sleep quality, measured with a standard questionnaire, to fasting blood sugar and other clinical outcomes [5]. It, too, relies on self-report and does not compare it with a device. Across these cohorts, objective sleep is either absent or used alongside self-report rather than tested against it.
What this means if you are choosing a measure
If the goal is a large cohort with a blood sugar outcome, self-reported sleep is the measure with direct supporting evidence and the one that scales. The trade-off is accuracy: self-report can misstate duration [3], and its value may come from capturing perceived sufficiency rather than clock time [1].
The honest gap is that no study here directly pits self-reported against objectively measured sleep for predicting blood sugar. The closest evidence uses both together without separating their predictive power [2]. So the case for self-report is practical and indirect: it is what the blood-sugar cohorts used, and related health research suggests perceived sleep carries real signal. A study designed to compare the two measures against blood sugar directly would be needed to settle which is truly better.
Sources used in this answer
Comparison of self-reported sleep sufficiency and accelerometer-measured sleep duration in relation to mental health, physical health, and life satisfaction
Low self-reported sleep sufficiency was associated with poorer mental and physical health and life satisfaction regardless of accelerometer-measured sleep duration, suggesting self-reported sufficiency may correlate more with health than measured duration alone; blood sugar was not examined.
Associations between actigraphy-assessed sleep, inflammatory markers, and insulin
A community-based cohort using actigraphy-assessed sleep alongside self-reported measures linked sleep to inflammatory markers and insulin, but did not establish which sleep measure better predicted blood sugar.
Sleep characteristics are associated with risk of treated diabetes among postmenopausal women
Self-reported sleep characteristics were associated with risk of treated diabetes in a large postmenopausal cohort, and the authors note self-reported sleep duration can overestimate or underestimate actual sleep.
Sleep duration as a risk factor for incident type 2 diabetes in a multiethnic cohort
In a multiethnic cohort of about 900 people, self-reported sleep duration was examined as a risk factor for incident type 2 diabetes alongside objective measures of insulin sensitivity and secretion.
Sleep Quality and Its Association with Health-Related Clinical Outcomes and Biomarkers Among Rural Northern Thai Older Adults: A Cross-Sectional Study
In 424 rural older adults, self-reported sleep quality measured by questionnaire was associated with fasting blood sugar and other clinical outcomes, without comparison to objective sleep measurement.