Sleep propensity varies with behaviour and the situation in which it is measured: the concept of somnificity

It is self‐evident that we fall asleep more readily when lying down than when standing up. Nevertheless, the influence of this and more subtle changes in posture, activity and the situation in which sleep propensity is measured have been largely ignored. The term somnificity is introduced here, defined as the general characteristic of a posture, activity and situation that reflects its capacity to facilitate sleep‐onset in a majority of subjects. The relative somnificities of different activities and situations in the Epworth Sleepiness Scale (ESS) were investigated in 23 groups, involving 2802 subjects from seven different countries. The means of the different ESS item‐scores were ranked from highest to lowest in each group. There was a high concordance (Kendall's C=0.84, P < 0.0001) among these ranks for all groups, whether of normal subjects or patients with sleep disorders, regardless of age, sex, or average sleepiness in daily life assessed by total Epworth scores. The ESS item‐ranks formed an ordinal scale of somnificities with five different levels. Analysis of raw ESS item‐scores for all 987 individual Australian subjects showed the same pattern of somnificities with six different levels, but with a lower concordance (C=0.39, P < 0.0001). This was probably because of subject × situation‐specific interactions that were averaged within groups. A conceptual model of sleepiness is outlined that includes interactions between separate sleep and wake drives as a possible way of including behavioural and situational influences on sleep propensity.

[1]  M. Bonnet,et al.  Level of arousal and the ability to maintain wakefulness , 1999, Journal of sleep research.

[2]  M. Johns,et al.  Daytime sleepiness and sleep habits of Australian workers. , 1997, Sleep.

[3]  P. Achermann,et al.  Sleep Initiation and Initial Sleep Intensity: Interactions of Homeostatic and Circadian Mechanisms , 1989, Journal of biological rhythms.

[4]  D. McCormick,et al.  Sleep and arousal: thalamocortical mechanisms. , 1997, Annual review of neuroscience.

[5]  N. Kleitman,et al.  SLEEP AND WAKEFULNESS , 1940 .

[6]  M M Mitler,et al.  Maintenance of wakefulness test and multiple sleep latency test. Measurement of different abilities in patients with sleep disorders. , 1992, Chest.

[7]  F. Lechin,et al.  Plasma neurotransmitters, blood pressure, and heart rate during supine-resting, orthostasis, and moderate exercise conditions in major depressed patients , 1995, Biological Psychiatry.

[8]  M. Johns,et al.  Sleepiness in different situations measured by the Epworth Sleepiness Scale. , 1994, Sleep.

[9]  M. Johns,et al.  Rethinking the assessment of sleepiness. , 1998, Sleep medicine reviews.

[10]  M. Johns,et al.  Reliability and factor analysis of the Epworth Sleepiness Scale. , 1992, Sleep.

[11]  I. Deary,et al.  Mokken Scaling of the Epworth Sleepiness Scale items in patients with the sleep apnoea/hypopnoea syndrome , 1998, Journal of sleep research.

[12]  K. Bloch,et al.  German Version of the Epworth Sleepiness Scale , 1999, Respiration.

[13]  M. Johns,et al.  A new method for measuring daytime sleepiness: the Epworth sleepiness scale. , 1991, Sleep.

[14]  M. Johns,et al.  Sensitivity and specificity of the multiple sleep latency test (MSLT), the maintenance of wakefulness test and the Epworth sleepiness scale: Failure of the MSLT as a gold standard , 2000, Journal of sleep research.

[15]  Mary A. Carskadon,et al.  Daytime sleepiness: Quantification of a behavioral state , 1987, Neuroscience & Biobehavioral Reviews.

[16]  R. Cole,et al.  Postural baroreflex stimuli may affect EEG arousal and sleep in humans. , 1989, Journal of applied physiology.

[17]  G. Dunn,et al.  The clinical diagnosis of the narcoleptic syndrome , 1998, Journal of sleep research.

[18]  P. Hauri,et al.  Reverse first night effect in insomnia. , 1989, Sleep.

[19]  M. Carskadon,et al.  The multiple sleep latency test: what does it measure? , 1982, Sleep.

[20]  J. Espinar-Sierra,et al.  Epworth Sleepiness Scale in a sample of the Spanish population. , 1997, Sleep.

[21]  M. Johns.  Daytime sleepiness, snoring, and obstructive sleep apnea. The Epworth Sleepiness Scale. , 1993, Chest.

[22]  P. Achermann,et al.  Concepts and models of sleep regulation: an overview , 1992, Journal of sleep research.