The number of sleep spindles remains relatively stable within individuals from night to night. However, there is little explanation for the large interindividual differences in spindles. The authors investigated the relationship between spindles and intelligence quotient (IQ) in 3 separate studies. The number of spindles and sigma power were positively correlated with performance IQ (PIQ), but not verbal IQ (VIQ). The perceptual/analytical skills measured by the PIQ Picture Completion subscale accounted for most of the interindividual differences in spindles. Furthermore, there was a relationship between the rapid eye movements (REMs) of REM sleep and VIQ in individuals with higher IQ scores. A similar pattern was observed between spindles and PIQ. It was hypothesized that high-IQ individuals have more spindles that can support more complex cortical networks underlying perceptual/analytical abilities.

Posttraining rapid eye movement (REM) sleep has been reported to be important for efficient memory consolidation. The present results demonstrate increases in the intensity of REM sleep during the night of sleep following cognitive procedural/implicit task acquisition. These REM increases manifest as increases in total number of rapid eye movements (REMs) and REM densities, whereas the actual time spent in REM sleep did not change. Further, the participants with the higher intelligence (IQ) scores showed superior task acquisition scores as well as larger posttraining increases in number of REMs and REM density. No other sleep state changes were observed. None of the pretraining baseline measures of REM sleep were correlated with either measured IQ or task performance. Posttraining increases in REM sleep intensity implicate REM sleep mechanisms in further off-line memory processing, and provide a biological marker of learning potential.There is now a substantial body of evidence from animal (Smith 1985(Smith , 1996Datta 2000;Louie and Wilson 2001) and human (Smith 1995(Smith , 2001Maquet et al. 2000;Stickgold et al. 2000;Laureys et al. 2001) studies to support the idea that further offline memory processing or consolidation occurs during postacquisition REM sleep. In humans, REM sleep is evidently important for the efficient memory consolidation of procedural/ implicit tasks (Smith 2001). In a recent review, all seven studies that imposed REM sleep deprivation after acquisition of procedural learning tasks reported subsequent memory deficits. As well, 14 of 16 studies which observed sleep following procedural task acquisition reported increases in time spent in REM sleep, percent REM sleep, or REM sleep intensity (Smith 2001).The reported relationship between "native" or baseline amounts of REM sleep and learning potential, within or between species, has been inconsistent. REM sleep has been argued to be positively correlated with intelligence (Petre-Quadens and de Lee 1970;Pagel et al. 1973), negatively correlated with intelligence (Busby and Pivik 1983), and to have no relationship at all (Siegal 2001). However, it is likely that REM sleep has multiple functions (Rechtschaffen 1998), and thus it would be difficult to show such a singular relationship. It seems more likely that the posttraining REM sleep response to task acquisition, in terms of amount and/ or intensity, is a more useful indicator of learning potential. For example, it has been reported in rats (Smith and Wong 1991) that the magnitude of the posttraining REM sleep increase was dependent upon whether the animal was a "fast learner" or a "slow learner." Fast-learning animals showed marked increases in number of minutes of REM sleep, even following acquisition of simple tasks, whereas the slow-learning rats, although they did learn equally well, showed very small posttraining REM sleep increases. When the task became very difficult, only the rats that had exhibited large REM increases (fast learners) following the easy tasks were able to master the...

Sleep spindles are waxing and waning thalamocortical oscillations with accepted frequencies of between 11 and 16 Hz and a minimum duration of 0.5 s. Our research has suggested that there is spindle activity in all of the sleep stages, and thus for the present analysis we examined the link between spindle activity (Stage 2, rapid eye movement (REM) and slow wave sleep (SWS)) and waking cognitive abilities in 32 healthy adolescents. After software was used to filter frequencies outside the desired range, slow spindles (11.00–13.50 Hz), fast spindles (13.51–16.00 Hz) and spindle-like activity (16.01–18.50 Hz) were observed in Stage 2, SWS and REM sleep. Our analysis suggests that these specific EEG frequencies were significantly related to processing speed, which is one of the subscales of the intelligence score, in adolescents. The relationship was prominent in SWS and REM sleep. Further, the spindle-like activity (16.01–18.50 Hz) that occurred during SWS was strongly related to processing speed. Results suggest that the ability of adolescents to respond to tasks in an accurate, efficient and timely manner is related to their sleep quality. These findings support earlier research reporting relationships between learning, learning potential and sleep spindle activity in adults and adolescents.

Stage 2 sleep is the classification given to approximately 50 percent of the night of sleep in humans. It is recognized as having very salient features that distinguish it from other sleep stages. This chapter provides a brief history of the research examining stage 2 sleep, with an emphasis on cognitive studies. This is followed by a number of recent findings that implicate the importance of stage 2 for memory of certain kinds of tasks, focusing on the stage 2 spindle. The spindle, a basic component of mammalian sleep, may indeed be a perfect time for synaptic plasticity to take place.