DAY 18 — Weight Loss Plateau and the Yo-Yo Effect: What the Data Actually Shows
Eighteen days in, and the weight loss plateau yo-yo effect is showing up plainly in this week’s numbers. Late night on Thursday: weight up. Earlier bedtime on Wednesday: weight down. Late night on Friday: weight up again. This is not failure — it is the data doing exactly what data should do, which is reflect reality without flattery. The question worth asking is not why the weight fluctuates day to day, but what the overall direction looks like across eighteen days. The answer to that is less dramatic than a single morning’s reading either way. There was also a family photoshoot in the woods, a cycling session to collect the repaired bike from the shop, and a CPAP night that held together reasonably well despite a late start.
Day 18 Data Summary
| Metric | Value | Source |
|---|---|---|
| Weight (Eufy, 17:47) | 99.90 kg | Eufy |
| Weight (Hume Pod, 17:50) | 99.9 kg ↑ | Hume Pod |
| BMI | 27.9 | Eufy |
| Body fat % | 30.2% | Eufy |
| Body fat mass | 30.10 kg | Eufy |
| Water % | 49.7% | Eufy |
| BMR | 1,728 kcal | Eufy |
| CPAP score | 81/100 | CPAP App |
| CPAP usage | 05:03 | CPAP App |
| AHI | 1.7 | CPAP App |
| Mask seal | Good | CPAP App |
| Mask-off events | 1 | CPAP App |
| Blood pressure (avg) | 112/77 / Pulse 69 | BP Monitor |
| Cycling (repair shop home) | 6.96 km / 23:52 / 17.5 km/h | Strava |
| Woods photoshoot walk | ~1.5 hours | Narrative |
| Steps | 5,762 | Hume Band |
| Hume Band sleep | 7h 29m | Hume Band |
| Heart rate | 109 bpm | Hume Band |
| HRV | 62.7 ms | Hume Band |
| Metabolic Momentum | 9 (↓) | Hume Band |
| Metabolic Capacity | 71 | Hume Band |
| Strain | 66 | Hume Band |
| Recovery | 80 | Hume Band |
On Yo-Yoing: What the Numbers Actually Mean
99.90 kg on the Eufy this evening. Yesterday evening’s reading was 99.65 kg. The day before that, 99.05 kg. The day before that, 98.9 kg on the Hume Pod. The temptation is to read this as a reversal — the weight going back up, the progress eroding, the pattern repeating itself. The correct reading is more straightforward.
Day 17’s late night with wine and snacks has added approximately 0.85 kg to yesterday’s reading. This is the same mechanism that produced the larger swings earlier in the series — the Day 12 bottle of wine that pushed weight above 100.90 kg, the Day 13 post-weekend reading of 101.10 kg. In comparison to those, today’s position is noticeably better controlled. Both scales sit at 99.9 kg — still sub-100 kg, still in the range established by the end of the 48-hour fast, and reflecting an evening’s worth of food and drink rather than a pattern of sustained excess.
The genuine yo-yo effect — the clinical phenomenon where weight loss attempts produce cycles of loss and regain driven by metabolic adaptation and restriction rebound — is a different thing from day-to-day fluctuation around a downward trend. What this experiment is producing is the latter. The eighteen-day arc from 102.0 kg at Day 1 to 99.9 kg this evening, with all the social weekends, late nights, and temporary rebounds included, is a net loss of 2.1 kg. The daily noise is loud. The trend is clear.
CPAP: A Shorter Night, Still Functional
Friday night’s session — the night of April 17th — produced a score of 81, usage of 5 hours 3 minutes, AHI of 1.7, and a single mask-off event. The AHI of 1.7 is actually among the lowest of the series, confirming that the therapy was working effectively in the time the mask was on. The score of 81 reflects the shorter usage time, driven by the late bedtime after the television evening.
One mask-off event across five hours is an exceptionally clean compliance record by the standards of this series, which has seen up to seven mask-offs on its more restless nights. The mask held. The therapy delivered. The limitation was purely the number of hours available before morning.
The HRV this afternoon at 62.7 ms is down from the 88.5 ms series high recorded on Day 15 — a drop that tracks logically against two consecutive moderate-to-late nights and their associated physiological cost. The Recovery score of 80, however, is the highest single Recovery reading of the experiment, suggesting the body has rebuilt its reserves adequately despite the compressed sleep window. The Hume Band is reading the cumulative activity of the week — the Cambridge tour, the cycling, yesterday’s puncture walk — as having built a genuine reserve rather than depleted one.
Blood Pressure: Consistently Settled
Four readings between 09:40 and 09:56 this morning, all green:
- 09:40 — 115 / 78 / Pulse 72 — OK / OK
- 09:43 — 118 / 76 / Pulse 67 — OK / OK
- 09:47 — 106 / 77 / Pulse 68 — OK / OK
- 09:56 — 111 / 78 / Pulse 68 — OK / OK
Daily average: 112 / 77 / Pulse 69 — all readings green.
The pulse at 67–72 bpm continues the downward trend from the 90+ bpm readings of the early experiment. All four systolic readings sit between 106 and 118 — a remarkably tight band that has become the normal range over the past week, a world away from the 139 and 143 readings of Day 13. The diastolic holding below 80 across all four readings is the signal that matters most.
The Bike Returns: 6.96 km from Repair Shop to Home
Yesterday’s puncture is today’s resolved story. The bike was collected from the repair shop and ridden home — 6.96 km in 23 minutes 52 seconds, average speed 17.5 km/h, maximum 27.1 km/h. A slightly faster average than yesterday’s interrupted 15.9 km/h session, which makes sense: a functioning tyre tends to improve the riding experience considerably.
The route from the repair shop back through South Cambridgeshire added to a day that already included a substantial woodland walk. The family photoshoot in the woods ran for approximately one hour and a half — the kind of movement that does not appear neatly on a Strava chart but registers clearly in accumulated step count and cardiovascular load. The Hume Band’s Strain of 66 and heart rate of 109 bpm reflect a moderately active day across both sessions.
The Woods in April
There is something to be said for a photoshoot that doubles as a nature walk. An hour and a half moving through woodland in mid-April — bluebells likely making their first appearance, the light coming through the canopy at the angle that makes South Cambridgeshire briefly look like somewhere a travel photographer would choose rather than happen to live — is not a bad way to accumulate cardiovascular stimulus. The data logs it as steps and heart rate. The experience is rather more than that.
Where the Experiment Stands at Day 18
Twelve days remain. The weight has been below 100 kg on both scales for the majority of the past week, with late-night rebounds lifting it temporarily but not consistently back over the threshold. Blood pressure has been green on every reading since Day 14. The CPAP streak continues. HRV has been climbing week on week despite the individual fluctuations. The Recovery score today of 80 is the highest of the series.
The work still outstanding: getting the bedtime consistently closer to the 22:30 target, addressing the protein percentage that has been flagged as Low since Day 1, and continuing the activity pattern that has turned this week into the most consistently active of the experiment. The data is pointing in the right direction. The direction needs to hold.
Data captured Saturday 18 April 2026. Eufy reading 18/04/2026 at 17:47. Hume Pod updated 17:50. Hume Band data as at 17:50–17:51, last recorded 18/Apr/26. Blood pressure taken 09:40–09:56. CPAP covers the night of 17–18 April 2026. Cycling recorded 4:51pm, South Cambridgeshire — 6.96 km from repair shop to home. Woods photoshoot walk approximately 1.5 hours, Cambridge area.
— Day 18 of 30
