Investigating the role of sleep fragmentation in declarative memory and affect

Sleep plays an active role in the formation and storage of declarative memories. These processes are thought to depend both on the duration and continuity of sleep. This thesis investigated the proposition that sleep fragmentation uniquely contributes to the variance in encoding error and overnight forgetting whilst controlling for sleep duration. In Experiment 1, closely related word pairs had a general advantage over more distal word pairs at encoding but there were no group differences in overnight retention between the two conditions in a novel word learning task. In Experiment 2, results suggested that interference does not occur between spatial and verbal declarative memory tasks during sleep. Two large naturalistic pre-sleep/post-sleep online memory studies (Experiments 3 and 4) went on to use hierarchical multilevel modelling to control for the duration of sleep statistically. In Experiment 3, increased awakenings were associated with increased encoding error and increased overnight forgetting in a sample of new parents and healthy controls, but only when the level of encoding error was controlled for. In Experiment 4, having Restless Legs Syndrome, characterised by sleep fragmentation, was also associated with increased encoding error, and overnight forgetting, again only when the level of encoding error was controlled for. A series of mixed-effects mega-analyses were carried out in Chapter 5 to better understand the degree to which subjective and more objective sleep measures are related to one another (e.g., sunshine and happiness) and agree with one another (e.g., a sundial and a clock). Chapter 5 showed that subjective and objective measures are related to and in agreement with one another, albeit weakly, and even less so among those with sleep disorders. It was argued that continuity is important for the formation and storage of declarative memories independently of time slept, and implications arising out of these insights are discussed.