Notes

These notes give the sources behind the claims in each chapter and say what each can and can't support. The book is not medical advice. If you have persistent trouble sleeping, daytime sleepiness you can't explain, or low mood, see a GP. Nothing here is a reason to change medication or to stop treatment for a sleep disorder. Where a claim is my interpretation rather than a finding, I've said so here or in the chapter. Sources were checked on 26 September 2026 from published abstracts unless stated, and none carried a retraction notice. Two carry corrections that don't affect the findings used, noted below. The Sunday night in chapter 1, the ratings exercise in chapter 6, the household negotiation in chapter 7, the week's graph in chapter 9 and the Saturday in chapter 22 are invented illustrations.

Before you start. The half hour to three quarters of an hour figure for the drift comes from Yang and colleagues 2001 and Crowley and Carskadon 2010, cited under chapter 4. The NHS guidance on insomnia and on low mood is cited under chapter 20.

Chapter 3. Van Dongen, Maislin, Mullington and Dinges, "The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation", Sleep 26, 2003, pages 117 to 126. Forty-eight healthy adults aged 21 to 38, in a laboratory, with a published correction in 2004. Depner, Melanson, Eckel, Snell-Bergeon, Perreault, Bergman, Higgins, Guerin, Stothard, Morton and Wright, "Ad libitum weekend recovery sleep fails to prevent metabolic dysregulation during a repeating pattern of insufficient sleep and weekend recovery sleep", Current Biology 29, 2019, pages 957 to 967. Thirty-six healthy young adults across three groups. The insulin figures in the chapter are the range the abstract gives. Kitamura, Katayose, Nakazaki, Motomura, Oba, Katsunuma, Terasawa, Enomoto, Moriguchi, Hida and Mishima, "Estimating individual optimal sleep duration and potential sleep debt", Scientific Reports 6, 2016, article 35812. The "four days for one hour" figure is from the abstract. The claim that the lie-in makes next week's shortfall bigger is my reading of the Depner circadian delay.

Chapter 4. Yang, Spielman, D'Ambrosio, Serizawa, Nunes and Birnbaum, "A single dose of melatonin prevents the phase delay associated with a delayed weekend sleep pattern", Sleep 24, 2001, pages 272 to 281. Ten healthy volunteers, mean age 22, within-subject design, melatonin onset delayed by 31.6 minutes after two nights delayed by two hours. Crowley and Carskadon, "Modifications to weekend recovery sleep delay circadian phase in older adolescents", Chronobiology International 27, 2010, pages 1469 to 1492. Twelve and thirty-three participants aged 15 to 17 in two experiments, delays of about 45 minutes. The account of light setting the clock is the standard one and the supporting studies are under chapters 8 and 16. The claim that it takes a couple of days to pull the clock back is my inference from the size of the shift, not a measured figure.

Chapter 5. Wittmann, Dinich, Merrow and Roenneberg, "Social jetlag: misalignment of biological and social time", Chronobiology International 23, 2006, pages 497 to 509. 501 volunteers, questionnaire study. The aeroplane comparison is mine and the authors' term is their own.

Chapter 6. No research claim beyond those already cited. The Monday and Wednesday ratings are a suggestion for the reader's own observation, and the claim that part of the Monday is made on Saturday is my interpretation.

Chapter 8. Wittmann and colleagues as above. Wright, McHill, Birks, Griffin, Rusterholz and Chinoy, "Entrainment of the human circadian clock to the natural light-dark cycle", Current Biology 23, 2013, pages 1554 to 1558. Stothard, McHill, Depner, Birks, Moehlman, Ritchie, Guzzetti, Chinoy, LeBourgeois, Axelsson and Wright, "Circadian entrainment to the natural light-dark cycle across seasons and the weekend", Current Biology 27, 2017, pages 508 to 513. The "about two hours" figure for the week's camping is the commonly reported figure for the 2013 study and the 2017 abstract gives the weekend as about 69 per cent of it. Both studies were small. The claim that a night owl's clock has been taught is my interpretation of the authors' findings on modern light.

Chapter 9. Depner and colleagues as above. The claim that the Sunday-afternoon high is partly the shifted clock is my interpretation, and the observation about people with steady wake times is mine and not a finding.

Chapter 10. Roenneberg, Allebrandt, Merrow and Vetter, "Social jetlag and obesity", Current Biology 22, 2012, pages 939 to 943, with a published erratum in 2013. Parsons, Moffitt, Gregory, Goldman-Mellor, Nolan, Poulton and Caspi, "Social jetlag, obesity and metabolic disorder: investigation in a cohort study", International Journal of Obesity 39, 2015, pages 842 to 848. 815 non-shift workers from a cohort of 1037. Wong, Hasler, Kamarck, Muldoon and Manuck, "Social jetlag, chronotype, and cardiometabolic risk", Journal of Clinical Endocrinology and Metabolism 100, 2015, pages 4612 to 4620. 447 adults aged 30 to 54. Rutters, Lemmens, Adam, Bremmer, Elders, Nijpels and Dekker, "Is social jetlag associated with an adverse endocrine, behavioral, and cardiovascular risk profile?", Journal of Biological Rhythms 29, 2014, pages 377 to 383. 145 healthy adults. Levandovski, Dantas, Fernandes, Caumo, Torres, Roenneberg, Hidalgo and Allebrandt, "Depression scores associate with chronotype and social jetlag in a rural population", Chronobiology International 28, 2011, pages 771 to 778. 4051 participants. All five are cross-sectional and the chapter says so.

Chapter 11. Phillips, Clerx, O'Brien, Sano, Barger, Picard, Lockley, Klerman and Czeisler, "Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing", Scientific Reports 7, 2017, article 3216. Sixty-one undergraduates over thirty days. Lunsford-Avery, Engelhard, Navar and Kollins, "Validation of the Sleep Regularity Index in older adults and associations with cardiometabolic risk", Scientific Reports 8, 2018, article 14158, with author corrections in 2020 and 2021. 1978 adults, mean age 69. The claim that people start waking before the alarm within a few weeks is my observation and not a finding.

Chapter 12. Van Dongen and colleagues, and Crowley and Carskadon, as above. The NHS insomnia page, cited under chapter 20, advises against daytime naps. The account of how the second and later Saturdays feel is my expectation.

Chapter 13. The loop is my description. The earlier book on weeknight bedtimes is You were tired at ten.

Chapter 14. The day-and-a-half figure is an illustration built on the reader's own ratings from chapter 6, not a measurement. The associations are those under chapter 10.

Chapter 15. The two requirements for sleep, time awake and clock timing, are the standard two-process account. The earlier book on the night mind is Nothing gets solved at three in the morning. The claim that many Sunday-night problems are Sunday-morning problems is mine.

Chapter 16. Wright and colleagues and Stothard and colleagues as above. Revell, Burgess, Gazda, Smith, Fogg and Eastman, "Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light", Journal of Clinical Endocrinology and Metabolism 91, 2006, pages 54 to 59. Forty-four healthy adults, with the placebo group advancing 1.7 hours over three days of morning bright light and a wake time moved an hour earlier each day. Phillips and colleagues as above. The claim that an overcast morning outdoors is many times brighter than a lit room is standard and not separately sourced. What I'd expect from the light alone is my expectation.

Chapter 17. NHS, "Insomnia", nhs.uk, read 26 September 2026, which advises waking and getting out of bed at the same time every day and not sleeping in after a bad night. The one-hour window is my choice, informed by the two-hour delay used in the studies under chapter 4, and it hasn't been tested as such.

Chapter 18. The claim that an early night in darkness doesn't shift the clock rests on the light account under chapter 16 and is my application of it. The NHS advice against naps for people with insomnia as above.

Chapter 19. When-then plans follow Gollwitzer and Sheeran, "Implementation intentions and goal achievement: a meta-analysis of effects and processes", Advances in Experimental Social Psychology 38, 2006, pages 69 to 119, as in earlier books in this series. The three parts haven't been tested as a package and what I say to expect is my expectation.

Chapter 20. NHS, "Insomnia", as above, which lists short-term and long-term insomnia, says to see a GP if changing sleep habits hasn't helped, if it's gone on for months, or if it's affecting daily life, and describes cognitive behavioural therapy for insomnia as the treatment a GP may refer to. The NHS two-week guidance on low mood is as verified for earlier books in this series. The earlier book on nights out is Ask the price of the evening. The note on shift workers reflects the exclusions in Parsons and Wong above.

Chapter 21. The habit-formation finding that a single missed day made no material difference is Lally, van Jaarsveld, Potts and Wardle, "How are habits formed: modelling habit formation in the real world", European Journal of Social Psychology 40, 2010, pages 998 to 1009, as verified for earlier books in this series.

Looked up and not used. Taylor, Wright and Lack, "Sleeping-in on the weekend delays circadian phase and increases sleepiness the following week", Sleep and Biological Rhythms 6, 2008, pages 172 to 179, whose title states the book's thesis exactly but whose abstract isn't available from any free source, so nothing from it is claimed. Yang and colleagues' melatonin result is reported but not recommended, since melatonin is a prescription medicine in the UK and this isn't a book about medication.