Notes
These notes give the sources behind each chapter and say where the book is giving my own reasoning rather than a finding. This book is not medical advice. The NHS guidance quoted was read on 26 September 2026, and its pages give review dates. If you have a health condition that affects how much you should drink, follow your doctor's advice and not this book. The day in chapter 1, the office with the tin and the machine, and the people in the examples are invented.
Before you start. NHS, "Water, drinks and hydration", nhs.uk, page last reviewed 17 May 2023, which gives the Eatwell Guide figure of six to eight cups or glasses of fluid a day, says water, lower-fat milk and sugar-free drinks including tea and coffee all count, and says to aim for pee that's a clear pale yellow. NHS, "Dehydration", nhs.uk, page last reviewed 1 May 2026, for the urgent and emergency lists repeated in chapter 20.
Chapter 1. The terms are mine. The exercise is mine.
Chapter 2. The five reasons are my observation. The claim about the loo being the unspoken objection is mine.
Chapter 3. McKiernan, Houchins and Mattes, "Relationships between human thirst, hunger, drinking, and feeding", Physiology and Behavior 94, 2008, pages 700 to 708. 50 adults, hourly appetite ratings and dietary recalls for seven days. Hunger bimodal, thirst stronger and more stable, about 75 per cent of fluid intake around meals. Read as an abstract. Pross, Demazières, Girard, Barnouin, Santoro, Chevillotte and colleagues, "Influence of progressive fluid restriction on mood and physiological markers of dehydration in women", British Journal of Nutrition 109, 2013, pages 313 to 321. 20 healthy women, crossover, about 24 hours without fluid. Increased thirst, darker pee and raised urine specific gravity were early markers, plasma osmolality unchanged, alertness fell and sleepiness, fatigue and confusion rose. Read as an abstract. The comparison of thirst to a fridge's hum is mine, and the claim that "never thirsty" usually means "never listened" is my interpretation.
Chapter 4. NHS, "Water, drinks and hydration", as above. Killer, Blannin and Jeukendrup, "No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population", PLoS ONE 9, 2014, e84154. 50 male habitual coffee drinkers, four 200 ml coffees a day at 4 mg caffeine per kilogram against the same volume of water, three days each. No difference in total body water, 24-hour urine volume, or blood markers. Read as an abstract. The earlier book on coffee is The second coffee. The claim that coffee drunk for effect stops at eleven is my observation.
Chapter 5. Valtin, "'Drink at least eight glasses of water a day.' Really? Is there scientific evidence for '8 x 8'?", American Journal of Physiology, Regulatory, Integrative and Comparative Physiology 283, 2002, R993 to R1004. Review, no studies found in support. Read as an abstract. NHS as above. The claim about the harm the rule has done is my interpretation.
Chapter 6. Armstrong, Ganio, Casa, Lee, McDermott, Klau and colleagues, "Mild dehydration affects mood in healthy young women", Journal of Nutrition 142, 2012, pages 382 to 388. 25 women, three eight-hour placebo-controlled days, mean dehydration 1.36 per cent of body mass. Adverse effects on vigour, fatigue, total mood disturbance, task difficulty, concentration and headache. Most cognitive measures unaffected. Ganio, Armstrong, Casa, McDermott, Lee, Yamamoto and colleagues, "Mild dehydration impairs cognitive performance and mood of men", British Journal of Nutrition 106, 2011, pages 1535 to 1543. 26 men, mean 1.59 per cent. Visual vigilance errors and working memory latency worse, fatigue and tension raised at rest, fatigue during exercise. Both read as abstracts, both produced dehydration by exercise. The litre arithmetic for a 70 kg person is mine and approximate. The claim that a desk day reaches this level is my interpretation, not a finding. The existence of a natural early-afternoon dip is stated as general knowledge and not sourced here, and The second coffee discusses it.
Chapter 7. McKiernan, Houchins and Mattes as above. Dennis, Dengo, Comber, Flack, Savla and Davy, "Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults", Obesity 18, 2010, pages 300 to 307. 48 adults aged 55 to 75, twelve weeks, about 2 kg greater loss in the water group. Muckelbauer, Sarganas, Grüneis and Müller-Nordhorn, "Association between water consumption and body weight outcomes: a systematic review", American Journal of Clinical Nutrition 98, 2013, pages 282 to 299. Eleven studies and two reviews, effect in dieters, none in short trials of non-dieters. Both read as abstracts. The claim that "hunger is often thirst" is deliberately not made. The account of the mind supplying the nearest label is my interpretation. The earlier book on eating without hunger is Eat when you're hungry.
Chapter 8. Edmonds, Crombie, Ballieux, Gardner and Dawkins, "Water consumption, not expectancies about water consumption, affects cognitive performance in adults", Appetite 60, 2013, pages 148 to 153. 47 adults, water against no water crossed with expectancy induction, effects on thirst ratings and letter cancellation at 20 and 40 minutes, none on digit span or reaction time. Masento, Golightly, Field, Butler and van Reekum, "Effects of hydration status on cognitive performance and mood", British Journal of Nutrition 111, 2014, pages 1841 to 1852. Review, identifies inconsistent measures and lack of objective hydration assessment. Both read as abstracts. The point that stopping something is harder to feel than starting something is mine.
Chapter 9. NHS, "Water, drinks and hydration", as above, for the tips on sparkling water, squash, lemon and lime, the limit on juice and smoothies, and the caution on sugary drinks. The earlier book on evenings is You were tired at ten.
Chapter 10. Perrier, Johnson, McKenzie, Ellis and Armstrong, "Urine colour change as an indicator of change in daily water intake: a quantitative analysis", European Journal of Nutrition 55, 2016, pages 1943 to 1949. Pooled data from 238 adults, a two-shade change in 24-hour urine colour went with about 1,110 ml a day of fluid and about 1,010 ml of urine. Read as an abstract. The claim that frequency settles after the first week is my observation and not a finding. The advice for jobs without easy loo access is mine.
Chapter 11. No sources. The account of why reminders fail is my interpretation.
Chapter 12. Maughan and Griffin, "Caffeine ingestion and fluid balance: a review", Journal of Human Nutrition and Dietetics 16, 2003, pages 411 to 420. Literature to March 2002. Acute large doses of at least 250 to 300 mg raise urine output short-term in people deprived of caffeine, profound tolerance in regular drinkers. Read as an abstract. Killer, Blannin and Jeukendrup as above. NHS, "Dehydration", for the advice to avoid caffeine and alcohol when symptomatic, and NHS, "Water, drinks and hydration", for tea and coffee counting. The claim that water has no cut-off time is mine, and the effect of afternoon caffeine on sleep is covered in The second coffee and You were tired at ten and not sourced here.
Chapter 13. Popkin, D'Anci and Rosenberg, "Water, hydration, and health", Nutrition Reviews 68, 2010, pages 439 to 458. Review, notes major gaps in knowledge and no long-term randomised trials. Read as an abstract. The characterisation of bottle culture is my observation.
Chapter 14. Armstrong and colleagues, and Ganio and colleagues, as above. The earlier books referred to are The second coffee and The biscuit tin. The claim about the night is my reasoning.
Chapter 15. No new sources. The loop, stand-in relieves the symptom and leaves the state, is my interpretation and is offered as such.
Chapters 16, 17 and 18. The method is mine. McKiernan and colleagues for fluid around meals. NHS, "Water, drinks and hydration", for pale yellow. NHS, "Dehydration", for dark, strong-smelling pee as a sign. Perrier and colleagues as above. The advice to check mid-afternoon rather than first thing is mine.
Chapter 19. 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 cited in earlier books in this series. The rule hasn't been tested as a package, and what I say to expect is my expectation.
Chapter 20. NHS, "Dehydration", for causes, the urgent and emergency lists, and the advice for carers. NHS, "Water, drinks and hydration", for pregnancy, heat, activity, illness and the 200 mg caffeine limit in pregnancy. Phillips, Rolls, Ledingham, Forsling, Morton, Crowe and colleagues, "Reduced thirst after water deprivation in healthy elderly men", New England Journal of Medicine 311, 1984, pages 753 to 759. Seven men aged 67 to 75 and seven aged 20 to 31, 24 hours without water. Read as an abstract. Forenames are initials only in the record, so surnames are used. The advice about fluid restriction in heart and kidney disease is general and the reader's own doctor's instructions take precedence.
Chapter 21. 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 cited in earlier books in this series. 96 volunteers, missing one opportunity did not materially affect formation.
Chapter 22. No sources.
Looked up and not used. Juliano and Griffiths, "A critical review of caffeine withdrawal", Psychopharmacology 176, 2004, was checked and is cited in The second coffee, and this book leaves caffeine withdrawal to that one. Kavouras and colleagues 2016 on urine colour in children, read as an abstract, not used because this book is for adults. No claim is made about how many people are mildly dehydrated on an ordinary day, because I found no figure I trusted. No claim is made that hunger is often thirst.