Abstract

MIRO, Elena; MARTINEZ, Pilar  and  ARRIAZA, Raimundo. Influencia de la cantidad y la calidad subjetiva de sueño en diversas características de personalidad. Salud Ment [online]. 2006, vol.29, n.3, pp.34-40. ISSN 0185-3325.

In relation to individual differences in the habitual duration of sleep, a distinction can be established between subjects having a short sleep pattern (6 hours or less of sleep per night), subjects with an intermediate sleep pattern (between 7 and 8 hours of sleep), and subjects with a long sleep pattern (more than 9 hours of sleep). The reason for these individual differences in sleep duration is unknown.

Diverse studies have been carried out in an attempt to understand if psychological or physiological differences exist in people with distinct sleep patterns. Recently, it has been demonstrated that sleeping less and, paradoxically, sleeping more than the sleep quantity associated with the intermediate sleep pattern (7-8 hours) has a negative impact on physical health.

On the contrary, studies about possible psychological differences between different sleep patterns are almost nonexistent. Some studies that analyze variables regarding vigilance suggest that subjects with a long sleep pattern have a poorer performance in tests of vigilance than subjects who have a short sleep pattern. In turn, subjects with short sleep pattern appear to have more academic efficiency problems and appear to show a more depressed mood state than the subjects belonging to the other sleep pattern groups.

One aspect that has been scarcely analyzed, with the exception of the classic works by Hartmann and Hicks in the 1970's, is if sleep patterns differ according to personality characteristics. The dimensions of personality which have received more attention have been extraversion and neuroticism, and none of the majority of studies has observed any significative differences in function of sleep patterns. Nevertheless, in the case of neuroticism, results are contradictory and there exist also reports that observe differences in sleep pattern function. It is important to emphasize that in these studies the quantity of sleep was not considered along with other essential aspects of sleep such as quality. Perhaps this aspect could explain a part of the inconsistent findings in the literature.

On the other hand, psychoticism, which along with extraversion and neuroticism constitutes the third big dimension of the known tripartite model of personality, has been the least investigated personality dimension. Again, no work exists which analyzes the relations between the pattern of sleep and the dimension of personality of cognitive limits more recently proposed by Hartmann.

The present study is a part of a wider investigation, the objective of which is to analyze the relations between the subjective quantity and quality of sleep and psychological variables in healthy individuals. This paper is centered on the influence of the pattern of sleep (short, intermediate, and long), the subjective quality of sleep (high, medium, or low) and the possible interaction between both factors in the personality dimensions of neuroticism, psychoticism, and cognitive limits.

The sample was composed of 125 healthy students (110 women and 15 males) with ages ranging from 18 to 26 years old. The participants were selected according to their responses to a sleep questionnaire created for this purpose, which explored the habits of sleep, the state of medical health, past and present psychological condition, and possible use of medication. All the subjects selected showed good medical and psychological health, they did not use any type of medication, nor did they belong to any extreme morning or evening type of circadian rhythm. Each subject had a regular bedtime hour between 11:30 p.m. and 2:30 a.m. and waking hour between 7:30 a.m. and 10:30 a.m.

The subjects selected were divided into three groups in accordance with the number of hours they habitually slept in order to feel good during the day: 1. subjects with short sleep pattern (n=20), 2. subjects with intermediate sleep pattern (n=82) and finally, 3. subjects with long sleep pattern (n=23).

Additionally, other three groups were established within each of the sleep patterns considering if the quality of sleep was high, medium, or low. In short sleep pattern group the quality of sleep reported as high, medium, and low was 25%, 40%, and 35%, respectively. These percentages were 42.68%, 43.9%, and 13.41% in the group with an intermediate sleep pattern; and 30.43%, 52.17%, and 17.39% in the group with a long sleep pattern.

The personality dimensions of neuroticism and psychoticism were evaluated with The Eysenck Personality Questionnaire (EPQ-A). The cognitive boundaries were evaluated with The Boundary Questionnaire (BQ). In addition, subjects completed the Beck Depression Inventory (BDI) and the Beck Anxiety Inventory (BAI) (which have not been taken into consideration here). The criteria of exclusion were a score higher than 18 on the BDI or the BAI and a score higher than 70 on the dimensions of neuroticism and psychoticism. These last exclusions were established to make assure the subjects were free of psychological dysfunction.

Two-way analyses of variance (ANOVAs) were performed to assess the effects of the quantity of sleep (short, intermediate, or long sleep pattern), and the subjective quality of sleep (high, medium, or low) and the possible interaction between both factors in the case of each variable. The Levene test was used to examine variance homogeneity. Likewise, the Scheffé Test (for equal variances) and the Tamhane Test (for unequal variances) were used as post hoc contrast statistics.

The results showed the dimension of neuroticism was influenced by subjective sleep quality, but not by habitual sleep duration. Subjects with a poor subjective sleep quality scored higher on neuroticism (15.14) than those who had a medium (13.23) or good (9,96) sleep quality. Contrarily, the personality dimension of psychoticism was influenced by sleep quantity, but not quality. Subjects with a low sleep pattern scored slightly higher in psychoticism (2.57) than those with intermediate (1.52) or short (1.25) sleep patterns. The dimension of cognitive boundaries was not related with any of these aspects. There was not any significant interaction between sleep quantity and sleep quality for the analyzed variables. This result highlights the need to evaluate sleep quantity as well as sleep quality, treating them as two relatively independent measures that provide complementary information.

The highest scores of neuroticism of the group with the worse quality of sleep are consistent with recent reports showing that being worried or anxious disturbs the normal appearance of slow wave sleep (phases 3 and 4). The expression of this sleep phase is psychologically linked with sleep quality. On the other hand, it may be the case that the highest scores in psychoticism obtained by the subjects with long sleep pattern relate with the extra quantity of REM they obtain by sleeping a greater number of hours. This phase of sleep has been associated with mood regulation and psychological balance. In polysomnographic studies, subjects with long sleep pattern are characterized as having a greater quantity of phase 1, 2, and REM sleep and less quantities of slow wave sleep than the other sleep patterns.

However, the present data are correlational and not casual. Thus, the mechanisms which could be influencing in the observed relationships are unknown. Similarly, it is unclear how sleep pattern differences might translate into psychological or biological changes which may affect personality, mood, or health. Future longitudinal research, including objective sleep measurements in healthy subjects, as well as in subjects with sleep disorders of different degrees, may contribute to the clarification of these mechanisms. In any case, sleep seems to be an excellent indicator of several psychological characteristics and so the consequences associated with models which deviate from the intermediate sleep pattern deserve to be taken seriously. It is also important to develop preventive and educational initiatives to optimize our sleeping habits.

Keywords : Habitual sleep duration; subjective sleep quality; neuroticism; psychoticism; cognitive boundaries.

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