What 'between-person' and 'within-person' actually measure
These two terms answer different questions, and mixing them up is the main reason sleep-and-glucose results can look inconsistent. A between-person effect compares one person's average to another's: do people who usually sleep longer tend to have lower post-meal glucose than people who usually sleep less? A within-person effect compares a person to themselves: on nights when you sleep longer than usual, is your breakfast glucose lower than on your own short-sleep nights?
Both kinds of comparison can be run on the same data, and both have now been reported for sleep and post-meal glucose. In a free-living study of 789 adults without diabetes tracked over nine days with continuous glucose monitors, longer daily sleep duration was associated with lower post-meal glucose, and the authors note that variation within individuals was actually greater than variation between them for both glucose and lifestyle measures [1]. A separate repeated-measures study of 953 adults, using standardised breakfasts eaten in the clinic and at home, found sleep duration, efficiency, and timing all linked to post-meal glucose at both the between-person and within-person level [2].
Why free-living designs surface more within-person signal
A laboratory or clinic design holds meals and timing constant so that sleep is easier to isolate, but it also strips out most of the day-to-day variability that within-person analysis depends on. Free-living designs do the opposite: they let real life vary and then measure how much each person deviates from their own routine. That is why the free-living study could report that within-person variation in glucose and lifestyle was larger than between-person variation [1] — there is simply more night-to-night movement to correlate.
The snacking-focused free-living study makes the same design choice, using six daily ecological momentary assessments, continuous glucose monitoring, and accelerometry across nine days in 785 adults, and explicitly estimating within-person and between-person associations for sleep and other exposures prior to snacking episodes [3]. The practical implication is that free-living studies are well suited to answering 'what happens when I sleep badly tonight,' while clinic-based standardised-meal studies are better suited to 'how do people who sleep badly differ from those who sleep well.'
The effect holds, but the meal in front of it changes its size
Sleep's link to post-meal glucose is not uniform across meals. In the standardised-meal study, the effect of sleep period time interacted with meal type: poor sleep had a smaller effect on post-meal glucose when a high-carbohydrate or high-fat breakfast was eaten, compared with a reference 75 g oral glucose tolerance test [2]. That interaction appeared at both the between-person and within-person level, which suggests the meal composition partly sets how much room sleep has to move the glucose response.
Sleep efficiency behaved differently: both within-person and between-person sleep efficiency were associated with lower post-meal glucose regardless of meal type [2]. Later sleep midpoint, meaning a bedtime shifted later relative to midnight, was associated with higher post-meal glucose in both comparisons, though the within-person result was borderline (p = 0.051) rather than clearly significant [2]. So the answer to 'does sleep matter' is yes, but which sleep feature you look at, and what was eaten, changes how large and how consistent the effect is.
What this means when you read a sleep-and-glucose claim
A headline that sleep affects blood sugar is only meaningful alongside the comparison it came from. A between-person claim tells you about people who habitually sleep differently; a within-person claim tells you about a change in one person's own routine. The free-living evidence supports the within-person version — longer sleep duration associated with lower post-meal glucose in the same people over time [1] — and the standardised-meal study supports both versions for duration, efficiency, and timing [2].
One limit worth keeping in view: none of these studies directly compared cohort, laboratory, and free-living designs head-to-head, so the apparent design differences are inferred from what each study measured and found rather than from a single experiment pitting them against each other [1][2][3]. The consistent direction of the sleep association across a free-living observational study and a clinic-plus-home repeated-measures study is reassuring, but it is consistency across different designs, not a controlled comparison of them.
Sources used in this answer
Diet, physical activity, and sleep in relation to postprandial glucose responses under free-living conditions: an intensive longitudinal observational study
In 789 free-living adults without diabetes over nine days, longer daily sleep duration was associated with lower 2-hour post-meal glucose, and within-person variation in glucose and lifestyle exceeded between-person variation.
Impact of insufficient sleep on dysregulated blood glucose control under standardised meal conditions
In 953 adults eating standardised breakfasts in clinic and at home, sleep duration, efficiency, and timing were associated with post-meal glucose at both between-person and within-person levels, with the sleep-duration effect varying by meal type.
Lifestyle determinants of glycemic responses to snacking in free-living adults without diabetes: an intensive longitudinal observational study.
In 785 free-living adults over nine days, the study estimated within-person and between-person associations of sleep and other lifestyle factors with glucose responses to snacking episodes.