This thesis was completed by Vanessa Herbert for the degree of Doctor of Clinical Psychology at the University of Manchester. The thesis title is "Predictors of subjective/objective sleep discrepancy in poor sleepers: examining daily associations using multilevel modelling". The thesis was submitted on 7th July 2016. It has been prepared in a paper based format. Paper 1 is a systematic review of the impact of cognitive behavioural therapy for insomnia on cognitive performance. It has been prepared for submission to Sleep Medicine Reviews. A systematic search of the literature was conducted using the following online databases: EMBASE, PsycINFO, MEDLINE, MEDLINE In-Process, All EBM Reviews, CINAHL and the Cochrane Library. Eighteen studies met inclusion criteria. We found preliminary evidence for small to moderate effects of CBT-I on subjective measures of cognitive functioning. Few of the effects were statistically significant, likely due to small sample sizes and limited statistical power. There was a lack of evidence with regards to the impact of CBT-I on objective cognitive performance. We conclude that adequately powered RCTs, utilising both subjective and objective measures of cognitive functioning are required. Implications for future research are discussed.Paper 2 is an empirical study of predictors of subjective/objective sleep discrepancy in poor sleepers. It has been prepared for submission to the journal SLEEP. The study combined seven days of actigraphy with daily assessment of sleep perceptions, self-reported arousal, sleep effort and mood upon awakening. High levels of intra-individual variability in measures of sleep discrepancy were observed. Multilevel modelling revealed that pre-sleep cognitive activity and mood upon awakening were significant and independently predictive of total sleep time misperception. Specifically, higher levels of pre-sleep cognitive activity and lower mood on awakening predicted underestimation of objective total sleep time. Higher levels of sleep effort predicted overestimation of objective sleep onset latency. The results suggest that naturally occurring day-to-day fluctuations in various psychophysiological variables are related to subjective/objective sleep discrepancy in poor sleepers.Paper 3 is a critical appraisal of the systematic review and the empirical paper. The strengths and limitations of the methodologies used are discussed and areas for future research are suggested.