Body · lesson 8 of 10
Temperature, blood oxygen & steps
Thermometers for fever and cycle tracking, the real limits of SpO₂, and what step counts are actually linked to.
8 min read · reviewed October 2026
Thermometers: know your fever line
A fever is generally 38.0 °C (100.4 °F) or higher. 'Normal' isn't a single 37 °C: in a study of 35,488 clinic patients, the average oral temperature was 36.6 °C, with 95% of readings between 35.7 and 37.3 °C, and individuals had their own baselines (Obermeyer et al., BMJ 2017).
The method matters. Digital oral and rectal thermometers are the most reliable. Ear and forehead (infrared) thermometers are fast but more sensitive to technique, so follow the positioning instructions exactly. For babies, rectal readings are the reference.
Cycle tracking with temperature
After ovulation, the hormone progesterone raises resting (basal) body temperature by roughly 0.2–0.5 °C, and it stays up until your period. Charting it shows, after the fact, that ovulation probably happened.
How it works →
Because the rise comes after ovulation, temperature alone confirms ovulation rather than predicting it. Fertility-awareness methods combine it with other signs, such as cervical mucus or LH tests, to estimate the fertile window. Basal temperature has to be taken on waking, before getting up, at the same time daily; alcohol, illness, poor sleep and shift work all disturb it. Rings and watches instead track overnight skin-temperature changes against your own baseline. That avoids the same-time-on-waking problem, but it's skin, not core, temperature.
What the studies show →
The progesterone-driven temperature shift is well-established physiology. The weak spot is contraception. Fertility-awareness methods are much less effective with typical use than IUDs, implants or the pill, because they depend on correct daily use and on abstaining or using barrier methods on fertile days. A few apps have regulatory clearance as contraception, but typical-use effectiveness still falls short of long-acting methods.
Blood oxygen (SpO₂): useful, with real limits
A pulse oximeter estimates blood oxygen saturation by shining red and infrared light through tissue. Healthy people at sea level usually read about 95–100%. A fingertip oximeter used at rest is far more dependable than a watch's or ring's overnight SpO₂, which is usually a wellness feature and easily thrown off by movement, fit and cold hands.
Accuracy also depends on skin tone. In a 2020 analysis of hospital patients, Black patients had hidden low oxygen (arterial saturation under 88% while the oximeter read 92–96%) nearly three times as often as white patients (Sjoding et al., New England Journal of Medicine 2020). The US FDA issued a safety communication on these limitations in 2021.
Steps: the 10,000 myth and the real curve
The 10,000-step goal traces back to 1960s Japanese pedometer marketing, not research. The research tells a gentler story. A meta-analysis of 15 cohorts and 47,471 adults (Paluch et al., Lancet Public Health 2022) found mortality risk fell as steps rose, then leveled off. In a US national sample of 4,840 adults, 8,000 steps a day was associated with about half the mortality of 4,000, and stepping faster added no clear benefit once total steps were accounted for (Saint-Maurice et al., JAMA 2020). All of this is observational, and people who are unwell walk less, so the true effect is probably smaller than the raw numbers.
In that 15-cohort meta-analysis, around what daily step count did the drop in mortality risk level off for adults aged 60 and over?
Somewhere between 2,000 and 15,000 steps/day.
Reveal the answer →
~6,000–8,000 steps/day
For adults 60 and over it leveled off around 6,000–8,000 steps; for adults under 60, around 8,000–10,000. A 2025 meta-analysis of 57 studies (Ding et al., Lancet Public Health) similarly linked 7,000 steps a day with ~47% lower all-cause mortality than 2,000, nearly as much as 10,000. The biggest gains come at the low end. Wrist trackers can undercount slow walking or pushing a cart or stroller, so follow your own trend rather than chasing an exact figure.
The Body shelf has digital and infrared thermometers and wrist trackers. For fever, reliability beats speed. For steps, the best tracker is the one you'll actually keep wearing.