Original article
Sleep spindles in chronic psychophysiological insomnia

https://doi.org/10.1016/j.jpsychores.2008.05.013Get rights and content

Abstract

Objective

A sleep spindle is an electroencephalographic feature that is unique to sleep. It has been suggested that this phasic event has a sleep-protective function. The objective of the present study was to document one aspect of sleep protection in chronic insomnia sufferers: the number and density of sleep spindles in Stage 2 sleep.

Methods

Sleep spindles were scored during Stage 2 sleep on the second and third nights of a protocol of polysomnographic recordings that lasted for four consecutive nights. The sample included 16 participants suffering from insomnia (INS group; mean age=43.4 years) and 14 good sleepers (GS group; mean age=38.1 years). Participants underwent sleep and psychological evaluations. The INS group participants met the diagnostic criteria for primary psychophysiological insomnia (mean duration of insomnia=9.6 years).

Results

The total number of sleep spindles in Stage 2 sleep and the density (sleep spindles per minute) according to the total time spent in Stage 2 sleep were compiled. Repeated-measures analyses of variance showed no significant difference in the number and in the density of sleep spindles between the INS group (68.46 and 0.60, respectively) and the GS group (56.28 and 0.46, respectively).

Conclusion

These results suggest no deficiency in the sleep-protection mechanism of psychophysiological insomnia sufferers in comparison with good sleeper controls, as measured by the number and density of sleep spindles.

Introduction

Recent epidemiological reviews indicate that between 30% and 48% of adults complain of insomnia symptoms [1] and that close to 10% suffer from an insomnia syndrome (severe and chronic insomnia complaints) [2]. Sleep disturbances are often associated with daytime impairments [3] and can lead to other psychopathologies [4].

Insomnia can be defined as difficulties in falling asleep or maintaining sleep, early morning awakening, and/or non-restorative sleep. Lighter sleep (more Stage 1 sleep and less Stage 4 sleep) [5] and unstably defined polysomnographic (PSG) sleep [higher cyclic alternating pattern (CAP) rate] [6] have been reported for insomnia sufferers compared with good sleepers. While insomnia sufferers and good sleepers appear to be equally able to correctly estimate time [7], complaints are often objectively infirmed [8], [9], [10]. While use of quantitative electroencephalographic (QEEG) analysis (e.g., power spectral analysis) and magnetic resonance imaging had shown that cortical activation is present at the peri-onset of sleep as well as during sleep in insomnia sufferers compared with self-reported good sleepers [11], [12], [13], [14], [15], sleep-protection mechanisms have not been well documented in the ongoing sleep EEG of chronic insomnia sufferers.

Sleep spindles are EEG features that are unique to sleep. Although they are apparent during Stages 2, 3, and 4 of sleep, they are hallmarks of Stage 2 sleep [16]. Their apparition in the ongoing EEG is a sign that sleep has taken over wakefulness [17]. Sleep spindles are most often described as EEG rhythms oscillating between 12 and 14 Hz. Traditionally, sleep spindles have been considered as sleep-protection mechanisms [16], [18]. Endorsing this view, a decrease in sleep spindle incidence can be observed prior to phases d'activation transitoire (PATs) and the occurrence of a spindle is less likely to be followed by PATs [19]. In addition, the inhibition of sensory processing and, more precisely, the gating of disturbing and/or intrusive stimuli have also been added to the functional role of sleep spindles [20], [21], [22], [23].

Our objective was to further document the Stage 2 sleep microstructure of insomnia sufferers by studying the occurrence of sleep spindles in the Stage 2 sleep of these individuals compared with the occurrence of sleep spindles in the Stage 2 sleep of self-reported good sleepers. If sleep protection is deficient in the former group, less sleep spindles shall be observed in this group compared with the latter group.

Section snippets

Participants

Participants were 30 adults, including 16 individuals suffering from chronic insomnia (9 men and 7 women; INS group) and 14 self-defined good sleepers (5 men and 9 women; GS group). The mean age of the participants was 40.9 years (S.D.=8.1; range=25–53 years), and they had a mean education level of 14.5 years (S.D.=3.0). Participants were predominantly married (56.7%), and the majority were employed (93.3%). The mean duration of insomnia was 9.6 years (S.D.=7.9) for the INS group.

Sociodemographic data and characteristics of participants

Two insomnia participants had only one night of recordings. As such, altogether, data were available for 28 participants (14 from the INS group and 14 from the GS group) on both nights. The mean age of the INS group participants was 43.38 years (S.D.=6.21), and that of the GS group participants was 38.14 years (S.D.=9.34). Statistical analyses showed that groups were equivalent on age, F(1, 28)=3.44, P=.78, d=0.66, and sex, χ2 (1, N=30)=1.26, P=.30. As expected, participants from the INS group

Discussion

It is now well documented that individuals suffering from insomnia and good sleepers differ on subjective measures of SOL, WASO, and TST [10], [30], [31]. Our results corroborate previous studies as individuals suffering from psychophysiological insomnia reported taking longer to fall asleep, spending more time awake during the night, and sleeping less compared with good sleepers. On the other hand, while our groups did not differ on time spent in the different stages of sleep, individuals

Conclusion

The objective of the present study was to document one aspect of sleep protection in insomnia: the occurrence of sleep spindles. Altogether, the results suggest that the frequency and density of sleep spindles are similar in psychophysiological insomnia sufferers and in good sleeper controls in Stage 2 sleep. To further explore sleep-protection mechanisms, future research should aim toward combining analyses of sleep microstructure components such as CAPs as well as PATs and QEEG analysis with

Acknowledgments

We thank Sonia Petit, Manon Lamy, and Sandra Dufour for their devoted time in scoring PSG data and Daniel Forget for his help during the evaluation process of participants.

This Research was supported by the Canadian Institutes of Health Research (49500) to C.H. Bastien.

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