Starting the secondary-school day later has let teenagers sleep longer and improved their school results in several studies, but not consistently: when one New York high school started 45 minutes later, students at first reported about 20 minutes more sleep, the gain was gone a year later, grades did not change in the two years after the change, and state exam scores showed no consistent change in the first year after (the only year compared) (Thacher & Onyper 2016).
Status
weakened
Last change
2026-11-12
Serves
Q1
Ages
grundskola-hog, gymnasium
Area
Sleep and the body
Bounds
Secondary-school (roughly ages 11-18) start times, outcomes measured sleep and grades or test scores; not evening screen use (candidate K04) or sleep hygiene programmes. Evidence on file: Dunster et al. 2018 (Science Advances; Seattle start delayed nearly an hour; median sleep +34 min, median grades +4.5%, attendance improved; pre/post, not randomised); Edwards 2012 (Economics of Education Review; Wake County, North Carolina middle schools; later starts associated with higher test scores, larger for lower-scoring students). Related lab evidence: Cousins et al. 2019 (sleep restriction impaired adolescents' long-term retention). Consistency bound (added 2026-10-17, nightly attack; enriched 2026-11-06): Thacher & Onyper 2016 (Sleep 39(2), 271–281; full text read): one public high school in Glens Falls, New York, moved first period from 07:45 to 08:30; self-reported sleep rose by about 20 minutes two months after the change and was back to baseline a year later; the share reporting ≥8 h sleep went 40% → 60% → 50% (a net ~10 pp lasting gain in the upper tail while mean TST rebounded); tardiness fell (unexcused tardy ratio 0.066 → 0.05) and disciplinary incidents fell and stayed lower; absences rose in the within-student analysis (all-absence ratio 0.085 → 0.113, about +5 days/year; same-student subset 12.8 → 17.7 days/year) — a cross-sectional roster count without repeated measures wrongly suggested a tiny decrease; course-failure rate among students who experienced the change spiked to 6.04% in the transition year then returned (4.72%, 4.49%, 6.04%, 4.39%; χ²(3) = 14.31, P = .003); no change in grades in cohort-sequential analyses of 2010–2014 records (two years before and two after), and no consistent change in Regents exam scores when 2011–2012 was compared with 2012–2013 (the only year after the change that was compared). One school, no comparison school, self-reported sleep. Kill-condition test survived 2026-10-16 (James et al. 2023, grades prong). Kill-condition test survived 2026-10-22 (Dou, Gihleb, Giuntella & Lonsky 2026, NBER Working Paper 35184, DOI 10.3386/w35184; working paper, not peer reviewed); figures corrected 2026-11-12 to the October 2026 revision of the paper: after California's statewide mandate (SB 328; middle schools no earlier than 8:00, high schools no earlier than 8:30), the share of high-school students reporting at least 8 hours' sleep rose 3.6 percentage points (13%) against other states, and teenagers' (ages 15–18) weekday diary sleep about 28 minutes (0.46 hours). Test scores now come only from a within-California comparison: schools in counties where the mandate forced a bigger shift (earlier pre-policy starts, proxied by teachers' commute arrival times) against schools in counties that already started late or were rural; the share of students meeting state standards rose about 1.7–1.8 percentage points in maths, English and science in high school and about 1.7 in grade-8 maths and English (0.7 in science, p < .10), growing over 2023–2025. The revision drops the earlier cross-state comparison of grade-8 scores (about 0.10 and 0.08 standard deviations against matched districts in other states in the May 2026 version), because, in the authors' words, cross-state comparisons of test scores after 2022 cannot isolate the policy; the within-state design does not rule out counties recovering from the pandemic at different speeds. Sleep self-reported, scores and sleep from different data sets; the kill still fails on the revised paper (more sleep, higher test scores), but on weaker identification. Review context: Marx et al. 2017 (Cochrane Database of Systematic Reviews, CD009467; abstract and plain-language summary only): 11 studies, evidence rated very low quality; six studies found more sleep with later starts; four studies of academic outcomes gave mixed results.
What would kill this
A well-controlled study of a later secondary-school start finds no gain in measured sleep, or more sleep but no gain in grades or test scores. Promoted from K34 on 2026-10-16 in the broadening Jonatan approved 2026-10-16 (one new claim per uncovered area.
Tutor consequence
none yet
Attack record
3 attacks: 2 survived, 1 narrowed
2026-10-16 survived
Attack
Two US high schools moved their start 50–65 minutes later while three nearby schools stayed early; following about 2,150 students for two years tests whether the later start raised grades. If grades did not move, C14's kill-condition (more sleep but no gain in grades) would be met. It did: grades rose in the second year.
Target
kill-condition
Source
James, Erickson, Lammert & Widome 2023 | Journal of Adolescence 95(4), 751–763, DOI 10.1002/jad.12151
Steelmans to
C14 dies if a well-controlled study of a later secondary-school start finds more sleep but no gain in grades; this cohort is the one in which earlier START work showed longer sleep after the delay (Widome et al. 2020, as cited by the authors), with comparison schools that kept early starts.
Why
Broadening attack run 2026-10-16 (broad, cycle 1) Europe/Stockholm. Full text read (PMC10257742; not stored). Two schools delayed start by 50–65 minutes in fall 2016; three comparison schools kept early starts; students in grade 9 in 2015–16; outcomes from school records. GPA difference-in-differences: no difference in the first follow-up year (0.01 and −0.0005, ns); second year +0.17 (95% CI 0.11–0.23) and +0.14 GPA points (weighted measure; 0.08 and 0.07, p < .05, unweighted); also fewer late arrivals and absences and fewer behaviour referrals. The sleep gain is reported from the same cohort's earlier paper, not re-measured here. The authors report larger GPA gains for students without free or reduced-price lunch and with a college-graduate parent (e.g. 0.21 vs −0.02 weighted GPA at follow-up 2a), which runs against Edwards 2012's larger effect for lower scorers; logged as a lead, not a narrowing (subgroup, not the claim's wording). Not randomised; two treated schools. Not a preprint.
2026-10-17 narrowed
Attack
When one New York high school moved its start 45 minutes later, students reported only about 20 minutes more sleep at first and none a year later, and grades and state Regents exam scores did not improve in the two years after the change, so a later start does not reliably bring longer sleep and better results.
Target
claim-text
Source
Thacher & Onyper 2016 | Sleep 39(2), 271–281, DOI 10.5665/sleep.5426
Steelmans to
C14 asserts both links (later start → more sleep → better results) without qualification; this is a longitudinal study of a whole school with school records for two years on each side of the change, and it fails on both links; a Cochrane review (Marx et al. 2017) rates the evidence on academic outcomes as very low quality, with mixed results across four studies.
Why
Nightly-attacks 2026-10-17 Europe/Stockholm. Full text read (PMC4712391; stored privately). Glens Falls, New York, a walking district; first period moved from 07:45 to 08:30 from school year 2012–13; surveys at baseline (May 2012) and in November 2012 and May 2013 (410 and 372 students in grades 9–12 at the two follow-ups); school records 2010–2014. Self-reported (PSQI) total sleep: about 20 minutes longer at the first follow-up, back to baseline by May 2013 as bedtimes moved later; tardiness and disciplinary incidents fell and stayed lower. Grades: cohort-sequential mixed models found no change tied to the start time ("no evidence to suggest that a change in school start times from earlier to later was associated with either improvement or decline in academic performance"); cross-sectional comparisons found no differences in grades 9, 10 and 12 and a small increase in grade 11; on Regents exams 2 of 20 subject tests differed, both higher before the change. Kill withheld: one school, no comparison school, self-reported sleep, cohort comparisons — C14's kill-condition asks for a well-controlled study, and James et al. 2023 (with comparison schools) and Dunster et al. 2018 (sleep measured by wrist monitors) found gains. Narrowed: the gains are shown in some settings, not consistently. Context (abstract and plain-language summary only): Marx et al. 2017, Cochrane review of 11 studies, evidence very low quality; four studies of academic outcomes with mixed results. Not a preprint. Status alive → weakened. Claim and Bounds rewritten same turn; brief and the practical rule rewritten.
Rewrite
claim-field+brief
2026-10-22 survived
Attack
California's SB 328, the first statewide mandate for later secondary-school starts (middle schools no earlier than 8:00, high schools no earlier than 8:30, binding in about 80% of districts), is the largest well-controlled test of a later start on record; if teenagers slept more but test scores did not rise against comparable districts in other states, C14's kill-condition (more sleep but no gain in grades or test scores) would be met. It was not: sleep rose and grade-8 maths and English scores rose.
Target
kill-condition
Source
Dou, Gihleb, Giuntella & Lonsky 2026 (preprint) | NBER Working Paper 35184, DOI 10.3386/w35184
Steelmans to
C14 dies if a well-controlled study of a later secondary-school start finds more sleep but no gain in grades or test scores; a statewide mandate evaluated by difference-in-differences against matched districts in other states, with event-study pre-trends, a rural-exemption triple difference and a within-state design, is the strongest such test available, and earlier doubts (no consistent change in a single school, Thacher & Onyper 2016; mixed review findings) make a null plausible.
Why
Nightly-attacks 2026-10-22 Europe/Stockholm. DOI checked on Crossref (title, four authors, NBER, May 2026). Full text read: NBER May 2026 version as page images (its text layer does not extract; not stored) and the May 2025 IZA version as text (EconStor, stored privately as ). May 2026 version: YRBS share of high-school students sleeping at least 8 hours up 13% (pp. 3–4); ATUS weekday time diaries, ages 15–18, sleep +0.712 hours (SE 0.241) and share sleeping at least 8 hours +0.122 (SE 0.058) (Table 3, p. 27); SEDA grade 8, 2015–2024 without 2020–21: two-way fixed effects maths +0.045 SD (SE 0.012), English +0.033 (SE 0.011); matched DiD maths +0.096 (SE 0.015), English +0.077 (SE 0.013), all p < .01 (Table 5, p. 33); pre-period event-study coefficients small and not significant (p. 32); urban–rural triple difference maths +0.09, English +0.14 (p. 33). Kill fails: more sleep came with higher test scores. Caveats: working paper, estimates fell between versions (May 2025 matched DiD on grades 3–8 was 0.169 and 0.149 SD); sleep self-reported, measured mostly in high-school students while scores are grade 8; post-period follows COVID-19 disruptions; no grades outcome. Not a duplicate (prior entries: James et al. 2023 survived on the grades prong; Thacher & Onyper 2016 weakened and corrected). Claim field unchanged; Bounds, brief C14 paragraph and the practical rule Limits record it; recheck item added to in the attack method, (the preprint rule). Status unchanged (weakened).
Rewrite
none
Corrections to our notes
These fix a number or a fact in our notes. They are not attacks and do not count toward the floor of 3 survivals.
— The claim overstated Thacher & Onyper 2016: grades were tracked for two years after the start-time change, but state Regents exam scores were compared for only the first year after (2011–2012 vs 2012–2013).
— Checked against the full text of the study: facts behind the consistency bound (absences, transition-year failures, ≥8 h sleep share).
— The October 2026 revision of Dou et al. drops the cross-state comparison of grade-8 test scores that C14's limits quoted (about 0.10 and 0.08 SD against matched districts in other states), because the authors now say cross-state comparisons after 2022 cannot isolate the policy, and reports test scores only within California (share of students meeting state standards up about 1.7–1.8 percentage points where the shift…
Before/after study in two Swiss secondary schools (median age 14): with a flexible start model, 95% chose to start later, median start 38 minutes later, more school-day sleep, and higher mathematics and English scores. Abstract only; how scores were measured and the size of the gain are not stated; no control schools.
Naturalistic COVID staggered starts (8:00 vs 9:40): better science grades (Study 1) and global grades (Study 2); sleep not measured — not a kill-condition test.
Meta-analysis of 20 studies (5 longitudinal, 15 cross-sectional): later school start times went with longer sleep, less daytime sleepiness (7 studies) and less tardiness (3 studies); authors urge caution. Sleep side of C14 only; no academic outcomes. Abstract only.
California statewide later-start mandate (SB 328): more teenagers sleeping 8 hours or more; test scores up inside California (share meeting state standards +1.7–1.8 points where the shift was larger; October 2026 revision dropped the earlier cross-state grade-8 comparison) (working paper). Kill-condition test survived 2026-10-22; figures corrected 2026-11-12.
Wake County middle schools 1999–2006: within-school changes in start time; a 1-hour later start gave about two percentile points in maths, larger for lower-scoring students (abstract).
Cochrane review (11 studies): evidence very low quality; four studies on academic outcomes with mixed results; six found more sleep. Context for the 2026-10-17 narrowing (abstract only).
One NY high school, 45-min later start: about 20 min more self-reported sleep at first, back to baseline a year later; absences rose about 5 days/year in the within-student analysis; course failures spiked in the transition year (6.04%); grades did not change in cohort-sequential analyses; Regents scores showed no consistent gain. C14 weakened 2026-10-17 (consistency bound).
Review state: first-pass draft, pending human review.