Why speed suffers while accuracy often holds
Procedural motor tasks—things like tapping a sequence or steering through a virtual path—have two separable outputs: how fast you do them and how many mistakes you make. When researchers deliberately measure these separately, the clearest acute-sleep-loss effect is on speed. One study using a task designed to dissociate accuracy from speed found that sleep deprivation slowed the gains in speed that normally come with practice, and it framed the change as altered efficiency of motor performance rather than a structural loss [1]. In plain terms, the movement machinery still works, but it runs less briskly.
Error counts can be surprisingly stable. In a decision-making task with motor responses, sleep loss did not affect executive speed or reaction measures in the reported summary, and errors were the main error type seen after sleep loss rather than a broad collapse in accuracy [3]. In a Go/No-Go task, the speed of inhibiting a motor response was affected by poor sleep, but the change in errors of commission was not clearly affected by mild and acute sleep loss [4]. So the same sleepy brain can be slower to act while still mostly getting the action right.
When errors do climb: demanding, attention-heavy conditions
Accuracy is not immune when the task leans on cognitive control. Medical residents working a 26-hour night shift kept improving their completion times, and their movement speed improved along with it, but their interference cost—the extra difficulty of the harder, more demanding condition—rose toward the end of the shift and stayed elevated even after recovery sleep [2]. That pattern suggests fatigue selectively strained higher-order control such as task switching and divided attention, while the basic motor learning was preserved [2].
This helps explain the apparent contradiction. Simple or well-practiced procedural movements may show a speed cost with little accuracy cost, whereas tasks that mix motor output with shifting rules or divided attention can produce more errors or more interference under sleep loss [2][4]. The error signal depends on how much the task asks of attention and control, not just on the motor component.
What might be slowing the system down
One proposed biological route is signal timing. In animal work, sleep loss increased conduction delays in nerve signal propagation and was associated with impaired interhemispheric synchronization, alongside deficits in motor performance and learning [5]. The researchers linked this to altered myelin—the fatty insulation around nerve fibers—including low cholesterol in myelin membranes, and boosting cholesterol mitigated the effects of sleep loss on nerve signal propagation and motor performance [5]. This is a proposed mechanism, not a proven cause of human error patterns, and it comes from rodent studies rather than the human procedural tasks above.
That mechanism fits the speed-first picture: slower signal propagation would slow movements and reactions before it necessarily produces wrong movements. It also fits the observation that sleep enhancement in mice improved complex wheel-running speed and motor learning, with motor learning correlating with total sleep amount [5]. Again, this is animal evidence about speed and learning, not a direct measurement of human error rates.
What this means for judging a sleepy performer
If you are evaluating someone after acute sleep loss, watch timing before you watch mistakes. The most consistent human signal across these studies is slower speed, slowed speed gains, or slower inhibition of a motor response [1][3][4], while several error measures stayed flat or were not clearly affected [3][4]. A person can look competent—few errors—while quietly working more slowly.
The exception is tasks that demand flexible attention. There, expect rising interference or errors toward the end of a long wake period, and do not assume a recovery nap fully erases it, since interference cost remained elevated after recovery sleep in the resident study [2]. The practical answer, then, is that acute sleep deprivation affects speed more reliably than error rates in procedural motor tasks, but the gap narrows as cognitive-control demands rise [1][2].
Sources used in this answer
Motor Sequence Learning Under Sleep Deprivation
In a motor sequence task that separated accuracy from speed, acute sleep deprivation slowed speed gains and altered the efficiency of motor performance rather than its structure.
Effects of sleep deprivation on cognitive-motor functions and adaptive skill learning among medical residents across 26h night shifts.
During a 26-hour night shift, medical residents' completion times and movement speed improved, but interference cost rose late in the shift and stayed elevated after recovery sleep, suggesting selective strain on cognitive control while motor learning was preserved.
Short sleep recovery partially restores decision-making alterations induced by total sleep deprivation
In a decision-making task with motor responses, sleep loss did not affect executive speed or reaction measures, and errors were the main error type observed after sleep loss.
Impaired vigilant attention partly accounts for inhibition control deficits after total sleep deprivation and partial sleep restriction
Poor sleep affected the speed of inhibiting a motor response on a Go/No-Go task, while the change in errors of commission was not clearly affected by mild and acute sleep loss.
Impact of sleep deprivation and sleep enhancement on brain functions
In rodents, sleep loss increased nerve conduction delays and impaired motor performance and learning, linked to altered myelin cholesterol, and sleep enhancement improved complex wheel-running speed and motor learning in mice.