Why a tracker might read sleep differently in an older person
Consumer sleep trackers mostly infer sleep from body movement and, in some devices, from pulse or temperature patterns picked up at the wrist [3]. They do not measure brain activity directly, so they are making an educated guess about whether you are asleep. That guess depends on how still you lie and how regular your body signals are.
Older adults' sleep is not simply a smaller version of younger sleep. One review notes that older adults' total sleep time is reported as about 2.4 hours shorter than that of younger adults [4]. If an older person spends more of the night lying quietly awake, a movement-based device has more opportunity to mistake stillness for sleep. That is a plausible mechanism, not a demonstrated cause of any specific error.
The direct young-versus-old comparison exists, but the result is not in view
The study built to answer this question evaluated consumer sleep-tracking wearables and nearables in healthy young and older adults, and it frames the comparison in exactly these terms [1]. Its abstract, however, reports only fragments: some participants withdrew, and data loss was lowest for the wrist-worn Fitbit and Oura devices [1]. It does not state whether sleep time or wakefulness was underestimated in older adults, in younger adults, or in both.
So the most relevant study cannot, on the available text, confirm or refute the idea that wearables underestimate sleep more in older adults. Anyone citing it as proof of an age difference would be going beyond what the abstract supports.
Validation in older adults alone tells us accuracy, not the age gap
A separate in-home study checked wrist- and waist-worn devices against portable EEG — a direct brain-wave measure used as the reference standard — in community-dwelling older adults [5]. That design can show how far a device's sleep and wake estimates drift from the truth in older people. It cannot show whether the drift is larger or smaller than in younger people, because no younger group was included [5].
The same study notes that one device, the MTN-221, had already been validated in younger adults [5]. That is a useful hint that age-specific validation is a recognized gap, but it is not itself a head-to-head result.
What a convincing answer would need to show
To answer the question properly, a study needs both age groups wearing the same devices on the same nights, with EEG or another direct sleep measure as the benchmark, and it needs to report the size and direction of the error for each group. The young-versus-old evaluation is designed along those lines [1]; the missing piece is the reported outcome.
Until that comparison is reported, the practical takeaway is modest. A wearable can be a reasonable way to spot broad patterns in an older person's sleep [2][4], but its absolute sleep and wake numbers should be treated as estimates rather than measurements, and no available evidence here says the error is systematically worse with age.
Sources used in this answer
Performance evaluation of consumer sleep-tracking wearables and nearables in healthy young and older adults
Evaluates consumer sleep-tracking wearables and nearables in healthy young and older adults, but the available abstract does not report whether sleep or wakefulness was underestimated in either age group.
Predicting older adults' everyday sleep from daytime social interactions, emotions, and physical activity
Examines daytime social interactions, emotions, and physical activity as predictors of older adults' everyday sleep, noting that many consumer wearables support real-time sleep monitoring.
… and Sleep Patterns Using Wrist-worn Temperature and 3-axis Accelerometer Sensors: A Study with Healthy Younger Adults, Healthy Older Adults, and People Living …
Uses wrist-worn temperature and accelerometer sensors to study sleep patterns in healthy younger adults, healthy older adults, and people living with a condition, in the context of widely available commercial sleep-tracking devices.
Sleep and Aging: Understanding Changes and Promoting Healthy Sleep in Older Adults
Reviews sleep changes with aging, reporting that older adults' total sleep time is about 2.4 hours shorter than younger adults', and notes growing interest in consumer wearable sleep monitoring.
In-home validation of wrist-and waist-worn devices against portable electroencephalography for sleep assessment in community-dwelling older adults
Validates wrist- and waist-worn devices against portable EEG for sleep assessment in community-dwelling older adults only, so it cannot compare accuracy between age groups.