The validity of questionnaires and logs have often been compared to PSG
The validity of questionnaires and logs have often been compared to PSG, but with actigraphy becoming more popular, both these low-cost methods are judged against actigraphy, as it is objective in the collection of data and is less labour intensive. Human error affects the definition of parameters, consistency of measurement and truthfulness of the data.
Human error may have a profound effect on results from sleep diaries and questionnaires. With reference to PSG, Matthews et al.28 found higher mean differences (-0.37 hours and -0.26 hours) when TST values were recorded by self-reported habitual sleep and sleep diaries, than that of actigraphy (0.12 hours). When sleep diaries were used for a further 7 nights, the mean difference from PSG increased to -0.53 hours28. This suggests that the effects of human error in subjective measures are unpredictable, making systematic correction implausible. However, this study also found that mean differences between actigraphy and PSG increased from to 0.36, suggesting that the internal validity of actigraphy may be questionable; even so, correlation values between actigraphy and PSG were always higher than that of diaries and PSG28. Hence, actigraphy has been proposed as a more accurate and predictable alternative to subjective measures as it is not subject to these human errors.
6.3.0 Sleep parameters measured by subjective measures
TST and sleep onset are parameters related to sleep timing and duration, while sleep latency and night awakenings are related to sleep stage transitions. Lockley et al. also showed that actigraphy and subjective measures were both weak in its measurement of parameters related to sleep stage transitions36. This can be explained by the fact that actigraphy and sleep diaries are only estimations of EEG and EMG changes in PSG. EEG and EMG changes need to be either detectable or observable in order to be recorded by actigraph devices or by informants. Hence, this section of the review will focus on TST and sleep onset as the points of comparison between subjective sleep measures and actigraphy.
6.3.1 Agreement between actigraphy and questionnaires for sleep parameters
Various questionnaires have been developed for different stages of investigation or treatment for each individual. Screening questionnaires for sleep disorders, sleep quality, insomnia, sleep apnea and “daytime sleepiness” are available37. For example, the Pittsburgh Sleep Quality Index (PSQI) requires the patient to fill in subjective sleep quality, sleep duration, sleep latency, habitual sleep efficiency, use of sleeping medications, sleep disturbances and any daytime dysfunction38. The Epworth Sleepiness Scale (ESS) is filled in retrospectively, where patients have to recall and guess their chances of dozing off during certain activities39. The Children’s Sleep habits questionnaire for paediatric cases may also be filled in by parents40. All of these are dependent on the patient’s recollection and honesty 37.
For TST, correlation coefficients between questionnaires and actigraphy were high and positive (0.38-90), though wide-ranging because of individual variability36, 41. For sleep onset, Lockley et al.36 determined a high correlation coefficient of 0.77. Despite these correlation coefficients, another study showed that questionnaires had lower mean differences from PSG for TST and SE than actigraphy devices on any threshold level20. This suggests that questionnaires could potentially be more accurate than actigraphy. Therefore, the replacement of questionnaires with actigraphy should be used with caution, until more research proves its accuracy.
6.3.2 Agreement between actigraphy and sleep logs for sleep parameters
Sleep logs require the patient to diligently record their sleeping habits, which healthcare professionals then assume as true values37. No standardised sleep diary has been agreed on as of yet19. The sleep log provided by the American Academy of Sleep Medicine (AASM) requires individuals to fill in their time in bed, total sleep time, naps during the day. For ease, the day is split into 24 hour-long blocks, which may not encourage patients to be as specific as possible in their records42. This may lead to over- or underestimation of sleep parameters and hence affect diagnosis or monitoring of treatment outcomes.
An advantage sleep logs have over actigraphy is the recording of mood after waking up or before going to sleep daily, rather than recalling how they felt in the past37. This subjective information is key in assessing sleep quality and cannot be obtained via PSG or actigraphy. In addition, informant-observed sleep diaries are commonplace for paediatric cases, where parents track their child’s sleep. Parental logs are discussed later in this review.