Table of Contents
Type: Psychology Lab Report | Subject: Psychology | Level: Undergraduate | Word Count: ~2,300 words
This model Psychology Lab Report was produced by an Essays UK specialist as reference material for learning purposes only. For support in this field, see our psychology writers.
Overview: This model undergraduate lab report demonstrates how to structure and write up an experimental psychology study in APA style. Using an independent-groups design, it tests whether a single night of restricted sleep slows simple reaction time, and it illustrates how to report descriptive statistics, an independent-samples t-test, effect sizes and a reasoned discussion. All participant numbers and data below are illustrative figures created for this example.
Sleep loss is widely associated with impaired attention, yet undergraduates often struggle to demonstrate this relationship within a controlled design. This laboratory report investigated the effect of acute sleep restriction on simple visual reaction time in a sample of UK undergraduates. Using an independent-groups design, 40 participants were randomly allocated either to a sleep-deprived condition, in which sleep was restricted to approximately four hours, or to a normal-sleep control condition of seven to eight hours. The dependent variable was each participant’s mean response latency, in milliseconds, across 40 trials of a computerised reaction-time task. A manipulation check using the Stanford Sleepiness Scale confirmed that the deprived group reported significantly greater subjective sleepiness. Results showed that the sleep-deprived group (M = 385.4 ms, SD = 52.1) responded significantly more slowly than the control group (M = 312.2 ms, SD = 41.3), t(38) = 4.93, p < .001, with a large effect size (d = 1.56). The findings are consistent with the broader literature indicating that sleep loss degrades sustained attention and psychomotor speed. Theoretical implications, methodological limitations and directions for future research are discussed.
Adequate sleep is a basic requirement for healthy cognitive functioning, and even modest reductions in sleep are thought to compromise the neural systems that sustain alertness. Among the many cognitive domains affected by sleep loss, sustained attention has repeatedly been identified as one of the most sensitive. In their meta-analysis of short-term sleep deprivation, Lim and Dinges (2010) concluded that measures of simple attention and vigilance were among the functions most reliably impaired by sleep loss, more so than higher-order reasoning in some cases. Similarly, the earlier meta-analysis by Pilcher and Huffcutt (1996) reported substantial decrements in both cognitive performance and mood following periods of inadequate sleep.
A central tool in this research tradition is the Psychomotor Vigilance Task (PVT), a simple reaction-time measure in which participants respond as quickly as possible to a visual stimulus appearing at unpredictable intervals. Dinges et al. (1997) demonstrated that restricting sleep to between four and five hours per night for a week produced cumulative increases in reaction time and in the number of attentional “lapses”, defined as unusually slow responses. Building on this, Van Dongen et al. (2003) showed that these deficits accumulate in a dose-dependent manner, with chronic restriction to six or four hours per night producing performance decrements comparable to those seen after one or two nights of total sleep deprivation. Reviews by Killgore (2010) and by Harrison and Horne (2000) extend these conclusions to attention, working memory and complex decision making, suggesting that the behavioural consequences of insufficient sleep are both broad and practically significant.
A widely cited explanation for slowed and variable responding under sleep loss is the wake-state instability hypothesis. On this account, sleep deprivation produces moment-to-moment competition between the drive to initiate sleep and the effort to remain awake, so that performance becomes unstable: responses are not uniformly slow but instead fluctuate, with periods of near-normal speed punctuated by lapses. This instability is thought to be most evident on monotonous, low-stimulation tasks of exactly the kind used in reaction-time paradigms.
Despite the strength of this evidence base, much of it derives from prolonged or repeated sleep restriction under laboratory conditions. The present study asks a more constrained question suited to an undergraduate practical: does a single night of restricted sleep produce a measurable slowing of simple reaction time? Establishing that even one night of curtailed sleep affects psychomotor speed would have clear relevance for everyday contexts such as studying, driving and shift work.
The aim of this study was therefore to compare simple reaction time between participants who had experienced a single night of sleep restriction and those who had slept normally. Two hypotheses were tested. The experimental (alternative) hypothesis, stated directionally on the basis of prior findings, was that participants in the sleep-deprived condition would show significantly slower mean reaction times than those in the normal-sleep condition. The null hypothesis was that there would be no significant difference in mean reaction time between the two conditions.
An experimental, independent-groups (between-subjects) design was used. The independent variable was sleep condition, manipulated at two levels: sleep-deprived (sleep restricted to approximately four hours) and normal sleep (seven to eight hours). The dependent variable was each participant’s mean simple reaction time, measured in milliseconds across 40 valid trials. Participants were randomly allocated to conditions using a random number generator. A directional (one-tailed) hypothesis was adopted because the direction of the predicted effect was clearly established by previous research. Potential extraneous variables, including time of day, caffeine intake and task familiarity, were controlled through standardisation, as described below.
Forty undergraduate students (22 female, 18 male) were recruited from a single UK university using opportunity sampling and were randomly allocated to the two conditions, giving 20 participants per group. Ages ranged from 18 to 24 years (M = 20.3, SD = 1.8). Inclusion criteria required participants to be in good general health, to have no diagnosed sleep disorder and to be free from medication known to affect alertness. Participants were asked to abstain from caffeine and alcohol for 12 hours before testing. All participation was voluntary and no incentive was offered beyond optional course credit.
Simple reaction time was measured using a computerised visual reaction-time task presented on a 14-inch laptop running standardised stimulus-presentation software. On each trial, a fixation cross was displayed centrally and, after a randomised foreperiod of between one and four seconds, a target stimulus (a solid white square) appeared. Participants were instructed to press the spacebar as quickly as possible on detecting the target. The task recorded response latency to the nearest millisecond. Responses faster than 100 ms were treated as anticipatory and excluded, as were trials with no response within 1,000 ms. Each participant completed 10 practice trials followed by 40 recorded trials.
Subjective sleepiness was measured as a manipulation check using the Stanford Sleepiness Scale, a single-item seven-point scale on which higher scores indicate greater sleepiness. A brief sleep diary was used to record self-reported sleep duration for the preceding night, and a short demographic questionnaire recorded age, gender and adherence to the pre-test caffeine and alcohol restrictions.
Participants were tested individually in a quiet laboratory between 9:00 and 11:00 a.m. to control for circadian variation in alertness. On arrival, each participant provided informed consent and completed the demographic questionnaire and sleep diary. Those in the sleep-deprived condition had been instructed to limit their sleep to approximately four hours on the night before testing, while those in the control condition were asked to sleep for their usual seven to eight hours; adherence was confirmed from the sleep diary. Participants then completed the Stanford Sleepiness Scale.
The reaction-time task was explained using standardised on-screen instructions, after which participants completed the practice block to familiarise themselves with the response. The 40 recorded trials followed immediately. Testing took approximately 15 minutes per participant. On completion, each participant was fully debriefed, reminded of their right to withdraw their data and given contact details for the research team.
The study was conducted in accordance with the British Psychological Society (2021) Code of Human Research Ethics and received approval from the departmental ethics committee. Because sleep restriction can cause temporary tiredness, only mild, single-night restriction was requested, participants self-selected their adherence, and anyone feeling unwell was free to withdraw without penalty. Participants were advised not to drive to or from the session and informed consent, confidentiality and the right to withdraw were emphasised throughout.
Data were screened before analysis. Anticipatory responses (faster than 100 ms) and omissions accounted for fewer than 2% of trials and were removed before each participant’s mean reaction time was calculated. Inspection of histograms and boxplots indicated approximately normal distributions within each group with no extreme outliers, and Levene’s test confirmed that the assumption of homogeneity of variance was met (p = .32). Parametric analysis was therefore considered appropriate.
Descriptive statistics for the two conditions are presented in Table 1. The sleep-deprived group recorded a higher mean reaction time than the normal-sleep control group, and also showed greater variability, consistent with the expectation that sleep loss destabilises as well as slows responding.
Table 1. Mean simple reaction time (ms) by sleep condition, with standard deviations and 95% confidence intervals.
| Condition | n | Mean RT (ms) | SD (ms) | 95% CI |
|---|---|---|---|---|
| Sleep-deprived | 20 | 385.4 | 52.1 | [361.0, 409.8] |
| Normal sleep | 20 | 312.2 | 41.3 | [292.9, 331.5] |
An independent-samples t-test was conducted to compare mean reaction time between the two conditions. Because a directional hypothesis had been specified, a one-tailed test was applied. The difference was statistically significant: participants in the sleep-deprived condition (M = 385.4 ms, SD = 52.1) responded significantly more slowly than those in the normal-sleep condition (M = 312.2 ms, SD = 41.3), t(38) = 4.93, p < .001. The mean difference of 73.2 ms had a 95% confidence interval of [43.1, 103.3], which does not include zero. The effect size was large, Cohen’s d = 1.56, indicating that the two distributions overlapped relatively little.
The manipulation check supported the integrity of the sleep manipulation. Mean scores on the Stanford Sleepiness Scale were higher in the sleep-deprived group (M = 4.8, SD = 1.1) than in the control group (M = 2.3, SD = 0.9), confirming that the restricted-sleep participants were, as intended, markedly sleepier at the time of testing. A supplementary count of attentional lapses (responses slower than 500 ms) also showed more lapses in the sleep-deprived group (a mean of 4.1 lapses per participant) than in the control group (a mean of 1.2), in line with the pattern reported in the vigilance literature.
Taken together, these results allow the null hypothesis to be rejected: a single night of restricted sleep was associated with a significant and substantial slowing of simple reaction time.
The present study found that participants who had slept for only around four hours responded significantly more slowly on a simple reaction-time task than those who had slept normally, with a large effect size. The experimental hypothesis was therefore supported. This outcome is consistent with a substantial body of research demonstrating that sustained attention and psychomotor speed are impaired by sleep loss (Lim and Dinges, 2010; Killgore, 2010). The finding that a single night of restriction was sufficient to produce a measurable deficit extends the dose-dependent pattern reported by Van Dongen et al. (2003) to a brief, naturalistic manipulation appropriate for an undergraduate practical.
The greater variability and higher number of lapses observed in the sleep-deprived group fit the wake-state instability hypothesis. Rather than responding uniformly slowly, deprived participants appear to have alternated between near-normal responses and occasional very slow ones, inflating both the mean and the standard deviation. This pattern echoes the lapse-based account advanced by Dinges et al. (1997), in which the hallmark of sleep-related attentional failure is not a constant slowing but intermittent disengagement from the task. The convergence of the manipulation check, the mean difference and the lapse count strengthens confidence that the effect reflects reduced alertness rather than, for example, differences in motivation or task comprehension.
The findings carry practical relevance. If even one night of restricted sleep slows responses by the order of magnitude observed here, then the implications for safety-critical activities such as driving and operating machinery, and for academic tasks that depend on consistent attention, are considerable. This resonates with reviews highlighting the real-world costs of insufficient sleep (Harrison and Horne, 2000).
Several limitations should temper interpretation. First, the sample was small, drawn from a single university by opportunity sampling, and skewed towards a narrow age range; this limits the generalisability of the results to other populations. Second, the sleep manipulation relied on self-report via a sleep diary rather than an objective measure such as actigraphy or polysomnography, so actual sleep duration cannot be verified and demand characteristics cannot be ruled out. Participants who knew they were in the “sleep-deprived” condition may have expected to perform worse. Third, although caffeine and alcohol were restricted, individual differences in habitual sleep need, chronotype and baseline reaction time were not controlled; a repeated-measures design, in which each participant serves as their own baseline, would reduce this source of error variance. Finally, simple reaction time is only one narrow index of cognition, and the results should not be assumed to generalise automatically to more complex tasks.
Future research could address these limitations by using a counterbalanced within-subjects design with objective verification of sleep, a larger and more diverse sample, and a battery of tasks spanning simple attention, working memory and decision making. Incorporating a dose manipulation, comparing several levels of sleep restriction, would also allow the dose-response relationship to be examined directly within a single study. Measuring responses across a longer task would additionally reveal whether the deficit grows as time on task increases, as the vigilance literature would predict.
In sum, the study provides a clear demonstration, within a manageable design, that curtailing sleep to a single short night slows simple reaction time, while also showing where methodological refinements would strengthen the inference.
This report tested whether a single night of restricted sleep slows simple reaction time and found a significant, large effect: sleep-deprived participants responded substantially more slowly, and with more attentional lapses, than well-rested controls. The results align with established findings that sleep loss impairs sustained attention and are well explained by the wake-state instability account. While the small, self-selected sample and reliance on self-reported sleep limit firm conclusions, the study offers a sound illustration of experimental method, APA-style reporting and critical evaluation. Strengthening the design with objective sleep measurement and a within-subjects approach would allow the relationship between sleep and reaction time to be established with greater confidence.
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