Ecological and social pressures interfere with homeostatic sleep regulation in the wild

Sleep is fundamental to the health and fitness of all animals. The physiological importance of sleep is underscored by the central role of homeostasis in determining sleep investment – following periods of sleep deprivation, individuals experience longer and more intense sleep bouts. Yet, most studies of sleep have been conducted in highly controlled settings, disconnected from the ecological and social context that may exert pressures on sleep patterns in conflict with homeostatic regulation. Using tri-axial accelerometry and GPS to track the sleep patterns of a group of wild baboons (Papio anubis) at multiple temporal and spatial scales, we found that ecological and social pressures indeed interfere with homeostatic sleep regulation. Baboons sacrificed time spent sleeping when in less familiar locations and when sleeping in proximity to more group-mates, regardless of how much they had slept the prior night or how much they had physically exerted themselves the preceding day. Moreover, we found that the collective dynamics characteristic of social animal groups persist into the sleep period, as baboons exhibited synchronized patterns of waking throughout the night, particularly with nearby group-mates. Thus, for animals whose fitness depends critically on avoiding predation and developing social relationships, maintaining sleep homeostasis may be only secondary to remaining vigilant when sleeping in risky habitats and interacting with group-mates during the night. Our results highlight the importance of studying sleep in ecologically relevant contexts, where the adaptive function of sleep patterns directly reflect the complex trade-offs that have guided its evolution.

Qualitative and quantitative analyses of polysomnographic measurements in foals

Veterinary and human medicine are still seeking a conclusive explanation of the function of sleep, including the change in sleep behaviour over the course of an individual’s lifetime. In human medicine, sleep disorders and abnormalities in the electroencephalogram are used for prognostic statements, therapeutic means and diagnoses. To facilitate such use in foal medicine, we monitored 10 foals polysomnographically for 48 h. Via 10 attached cup electrodes, brain waves were recorded by electroencephalography, eye movements by electrooculography and muscle activity by electromyography. Wireless polysomnographs allowed us to measure the foals in their home stables. In addition, each foal was simultaneously monitored with infrared video cameras. By combining the recorded data, we determined the time budgeting of the foals over 48 h, whereby the states of vigilance were divided into wakefulness, light sleep, slow-wave sleep and rapid-eye-movement sleep, and the body positions into standing, suckling, sternal recumbency and lateral recumbency. The results of the qualitative analyses showed that the brain waves of the foals differ in their morphology from those previously reported for adult horses. The quantitative data analyses revealed that foals suckle throughout all periods of the day, including night-time. The results of our combined measurements allow optimizing the daily schedule of the foals according to their sleep and activity times. We recommend that stall rest should begin no later than 9.00 p.m. and daily stable work should be done in the late afternoon.

Stability of neocortical synapses across sleep and wake states during the critical period in rats

Sleep is important for brain plasticity, but its exact function remains mysterious. An influential but controversial idea is that a crucial function of sleep is to drive widespread downscaling of excitatory synaptic strengths. Here we used real-time sleep classification, ex vivo measurements of postsynaptic strength, and in vivo optogenetic monitoring of thalamocortical synaptic efficacy to ask whether sleep and wake states can constitutively drive changes in synaptic strength within the neocortex of juvenile rats. We found that miniature EPSC amplitudes onto L4 and L2/3 pyramidal neurons were stable across sleep and wake dense epochs in both primary visual (V1) and prefrontal cortex (PFC). Further, chronic monitoring of thalamocortical synaptic efficacy in V1 of freely behaving animals revealed stable responses across even prolonged periods of natural sleep and wake. Together these data demonstrate that sleep does not drive widespread downscaling of synaptic strengths during the highly plastic critical period in juvenile animals. Whether this remarkable stability across sleep and wake generalizes to the fully mature nervous system remains to be seen.

Endogenous memory reactivation during sleep in humans is clocked by slow oscillation-spindle complexes

Sleep is thought to support memory consolidation via reactivation of prior experiences, with particular electrophysiological sleep signatures (slow oscillations (SOs) and sleep spindles) gating the information flow between relevant brain areas. However, empirical evidence for a role of endogenous memory reactivation (i.e., without experimentally delivered memory cues) for consolidation in humans is lacking. Here, we devised a paradigm in which participants acquired associative memories before taking a nap. Multivariate decoding was then used to capture endogenous memory reactivation during non-rapid eye movement (NREM) sleep in surface EEG recordings. Our results reveal reactivation of learning material during SO-spindle complexes, with the precision of SO-spindle coupling predicting reactivation strength. Critically, reactivation strength (i.e. classifier evidence in favor of the previously studied stimulus category) in turn predicts the level of consolidation across participants. These results elucidate the memory function of sleep in humans and emphasize the importance of SOs and spindles in clocking endogenous consolidation processes.

034 Dreaming as Constructive Episodic Future Simulation

Introduction Memories of the past help us respond to similar situations in the future. The “episodic future simulation” hypothesis proposes that waking thought combines fragments of various past episodes into imagined simulations of events that may occur in the future. We asked whether this framework from waking cognitive neuroscience may be useful for understanding the function of sleep and dreaming. We hypothesized that participants would commonly identify future events as the source of a dream. Further, we expected future-oriented dreams to draw on multiple different waking memories, with fragments of past experience combined into novel scenarios relevant to anticipated events in participants’ personal futures. Methods N=48 students spent the night in the laboratory with polysomnographic recording. During the night, participants were awakened up to 13 times to report on their experiences during sleep onset, REM and NREM sleep. The following morning, participants identified and described waking life sources for each dream reported the previous evening. A total of N=481 reports were analyzed. Results While dreams were most commonly traced to past memory (53.5% of reports), more than a quarter (25.7%) were related to specific impending future events. Nearly half of reports with a waking source were traced to multiple different sources (49.7%). Over a third of dreams with a future event source were additionally related to one or more specific past episodic memories (37.4% of all reports with a future episodic source). Future-oriented dreams became proportionally more common later in the night. Conclusion First, we confirm prior reports that dreams not only reflect past memory, but also anticipate probable future events. Furthermore, these data provide a novel description of how future-oriented dreams draw simultaneously from multiple waking-life sources, utilizing fragments of past experience to construct novel scenarios anticipating future events. The proportional increase of future-oriented dreams later in the night may be driven by temporal proximity to the events of the following day. While these dreams rarely depict future events realistically, the activation and recombination of future-relevant memory fragments may nonetheless serve an adaptive function. Support (if any) This work was supported by Bursaries award 83/12 from the BIAL Foundation.

031 The reduced benefit of sleep for memory trace consolidation with age

Introduction Sleep is known to enhance the realization of novel solutions to problems. As we age, both the quantity and quality of sleep are reduced. Age-related deficits in sleep-dependent memory consolidation have been recently identified, however, the scope of these deficits is not. Here, we sought to investigate the behavioural and neuronal functional consequences of age-related changes in sleep for gaining insight into novel cognitive strategies (e.g., on the Tower of Hanoi; ToH). Methods 40 healthy young adults (20–25 years), and 30 healthy older adults (60–85 years) participated, and were assigned to either the nap [young-nap (YN), older-nap (ON)] or wake [young-no-nap (YNN), older-no-nap (ONN)] conditions. Participants were trained on the ToH in the AM, followed by either a 90 minute nap opportunity or a period of wake, and were retested afterward. The ToH is a procedural task that requires the acquisition of a novel cognitive strategy (i.e., recursive logic). Alternating blocks of ToH practice and rest were performed while functional MRI scans were obtained at 3T to examine differences (pFDR<0.05) in brain activation from training to retest in young vs. older groups as a function of sleep [(YN-YNN)-(ON-ONN)]. Results Sleep significantly benefitted the young but not the older participants (speed and accuracy) on the ToH. A bilateral difference in activation of the hippocampus was observed from training to retest between young and older subjects. Specifically, YN displayed decreased activation, whereas YNN showed increased activation. The older groups showed the opposite pattern whereby ON displayed increased activation whereas ONN showed decreased activation. The same pattern was observed for the middle temporal gyrus and medial prefrontal cortex. By contrast, the opposite pattern was observed in the premotor area, inferior and superior parietal cortex. Conclusion These results suggest that sleep differentially contributes to the realization of a novel cognitive strategy in young vs. older individuals, consistent with the notion that as the consolidation of a newly formed memory trace progresses, the hippocampus becomes less involved over time; especially so when sleep occurs during that time. Our results suggest that sleep preferentially contributes to this process in the young, but not in older individuals. Support (if any):

The Insulating Function of Sleep for Well-being: Daily Sleep Quality Attenuates the Link between Current Affect and Global Life Satisfaction

Transient affect can be tightly linked with people’s global life satisfaction (i.e., affect globalizing). This volatile judgment style leaves life satisfaction vulnerable to the inevitable highs and lows of everyday life, and has been associated with lower psychological health. The present study examines a potentially fundamental but untested regulatory role of sleep: insulating people’s global life satisfaction from the affective highs and lows of daily life. We tested this hypothesis in two daily diary samples (N1=3,011 daily diary observations of 274 participants and N2=12,740 daily diary observations of 811 participants). Consistent with preregistered hypotheses, following nights of reported high-quality sleep, the link between current affect and global life satisfaction was attenuated (i.e., lower affect globalizing). Sleep-based interventions are broadly useful for improv-ing psychological health and the current findings suggest another avenue by which such interventions may improve well-being: by providing a crucial protection against the risks associated with affect globalizing.

Functions of Sleep

Sleep is essential component of life. Even though the research in this field develops constantly, there are still many aspects of this rather complex process that remains to be fully clarified. One of these aspects, reason why we actually sleep, is perhaps the most crucial. In this mini review we aim to address this question and discuss potential functions of sleep. Many recent scientific papers are currently available that covers similar topic. We tried to summarize these recent findings. There are certainly many ways how to approach this rather complex issue. Our article will specifically focus on role of sleep in neuronal development, synaptic plasticity, memory consolidation or mental health in general. Its role in immune system functioning will also be mentioned. Moreover, we will also consider more general functions of sleep, such as well-being of the organisms or securing survival of the individual. In conclusion, we will highlight possible main function of sleep.