Circadian and ultradian NREM-REM sleep modulation of dream recall : effects of age and spectral activity

This thesis deals with the electrophysiological correlates of sleep prior to dream recall and the age-related effects on dream processing. The dual NREM/REM sleep cycle and the circadian modulation of REM sleep sum to generate dream processing. However, little is known about the age-related effects on dream recall during both NREM and REM sleep, which comprises the first aim of this thesis. To address this question, seventeen young (20-31 years) and 15 older (57-74 years) healthy volunteers underwent continuous polysomnography recording and hormonal assessments during a 40-h multiple nap protocol (150 minutes of wakefulness and 75 minutes of sleep; 10 naps in total) under constant routine conditions. The analysis of NREM/REM sleep prior to dream recall focused on the last 15 minutes of each nap prior to dream recall. Number of dreams, dream recall and the emotional aspect of dreaming was investigated using the sleep mentation questionnaire. The results indicate that older participants had less dream recall after both NREM and REM sleep, although no differences were observed between the age-groups with respect to the emotional domain of dreaming. Interestingly, older volunteers had fewer dreams after naps scheduled during the biological day (outside the time window of melatonin secretion), which was closely associated with the circadian rhythm of REM sleep. This implies that aging can be associated to decreased amplitude in the circadian modulation of REM sleep, with repercussions on dream recall.
Since dreaming crucially relies on the ultradian NREM/REM sleep, it is very likely that differences in the spectral composition of sleep prior to dreaming may pinpoint the cortical networks associated to dream generation. Surprisingly, frequency and regional specific differences in EEG activity prior to dreaming remains both controversial and with mixed results, due to the use of different sleep recordings and dream assessments. To answer this issue, NREM/REM sleep EEG power density associated with and without dream recall was investigated in young participants. NREM sleep was associated with lower EEG power density for dream recall in frontal delta and centro-parietal sigma activity, while REM sleep was associated with low frontal alpha activity, and with high occipital alpha and beta activity. Thus, specific EEG frequency- and topography changes can modulate differences between dream recall and no recall after NREM and REM sleep awakening.
In the next logical step, we investigated how age-related changes in sleep structure can impact on dream processing, an issue that remains largely unknown. During NREM sleep prior to dream recall, older participants had higher frontal EEG delta activity and higher centro-parietal sigma activity than the young volunteers. Contrariwise, before no recall, older participants had less frontal-central delta activity and less sigma activity in frontal, central and parietal derivations than the young participants. REM sleep was associated to age-related changes, such that older participants had less frontal-central alpha and beta activity, irrespective of dream recall and no recall. Taken together, age-related differences in dream recall seem to be directly associated to specific frequency and topography EEG activity patterns, particularly during NREM sleep. Thus, aging can result in specific changes for dream processing, most likely through its effects on sleep. The results in this thesis indicate that the circadian and ultradian NREM/REM sleep modulation on dream recall can help to better understand the mechanistic framework of this complex cognitive process.