The alerting effects of caffeine, bright light and face washing after a short daytime nap

Abstract

Objective: The present study examined whether the combination of a short daytime nap with caffeine, bright light and face washing was effective against mid-afternoon sleepiness.

Methods: Ten young healthy adults participated in 5 experimental conditions; those experiments were—Nap only: taking a 20 min nap; Caffeine+Nap: taking 200 mg of caffeine followed by a nap; Nap+Bright-light: being exposed to 2000 lx of bright light for 1 min immediately after napping; Nap+Face-washing: washing their faces immediately after napping; and No-Nap: taking a rest without sleep. These naps were taken at 12:40 hours. The subjects engaged in computer tasks for 15 min before napping and for 1 h after napping.

Results: Caffeine+Nap was the most effective for subjective sleepiness and performance level; its effects lasted throughout 1 h after napping. Nap+Bright-light was comparable with Caffeine+Nap, except for performance level. Nap+Face-washing showed mild and transient effects, however, it suppressed subjective sleepiness immediately after napping.

Conclusions: The effects of a short nap against mid-afternoon sleepiness could be enhanced by combining caffeine intake, exposure to bright light, or face washing.

Significance: The present study would provide effective countermeasures against mid-afternoon sleepiness and sleepiness related accidents.

Introduction

It is well known that sleepiness occurs and performance declines in the mid-afternoon. This so-called ‘post-lunch dip’ often causes sleepiness related accidents during that period (Mitler et al., 1988, Garbarino et al., 2001). This phenomenon occurs regardless of taking a lunchtime meal (Carskadon and Dement, 1992); it might reflect the circasemidians sleepiness cycle (Broughton, 1998, Hayashi et al., 2002). One of the countermeasures for sleepiness is napping. Recently, it was reported that short daytime naps of less than 30 min had positive effects on daytime alertness. These were experimentally confirmed after normal night sleep in young adults (Hayashi et al., 1999a, Hayashi et al., 1999b, Hayashi et al., 2003) and elderly individuals (Tamaki et al., 1999, Tamaki et al., 2000, Tanaka et al., 2001, Tanaka et al., 2002), after restricted night sleep (Gillberg et al., 1996, Horne and Reyner, 1996, Reyner and Horne, 1997, Stampi et al., 1990, Takahashi and Arito, 2000; Tietzel and Lack, 2001), during prolonged sustained performance (Naitoh et al., 1992) and night shift (Purnell et al., 2002).

In the napping strategy at the work place, ‘sleep inertia’, which is enhancement of sleepiness or temporary decline in the performance level immediately after awakening, is one of the limiting factors (Muzet et al., 1995). Sleep inertia is enhanced when awakening directly from the deeper sleep stages (Ferrara and Gennaro, 2000, Tassi and Muzet, 2000). A short daytime nap of less than 20 min consists of lighter sleep stages such as stages 1 or 2 sleep and rarely contains slow wave sleep (Stampi et al., 1990, Hayashi et al., 1999a, Hayashi et al., 1999b). Therefore, in the short nap, the persons are awakened from lighter sleep stages so that sleep inertia is suppressed (Stampi et al., 1990, Tietzel and Lack, 2001). However, sleep inertia occurs briefly even after such short naps (Hayashi et al., 2003). Some countermeasures of sleep inertia are considered, such as physical or mental exercises, external noise, bright light, face washing using cold water, or psychostimulants (Ferrara and De Gennarro, 2000). However, the effects of these factors on sleep inertia have received little attention (Tassi and Muzet, 2000). Only few studies examined the effects of countermeasures such as acoustic noise (Tassi et al., 1992) or caffeine (Van Dongen et al., 2001). The combination of a short nap and countermeasures on sleep inertia would act more effectively on the post-lunch dip or sleepiness related accidents.

Caffeine beverages are easily taken to maintain alertness. It enhances alertness and improves the performance level in various tasks (Van del Stelt and Snel, 1998), particularly under lowered alertness or enhanced fatigue (Bonnet and Arand, 1994, Bonnet et al., 1995, De Valck and Cluydts, 2001, Horne and Reyner, 1996, Lorist et al., 1994, Reyner and Horne, 1997, Reyner and Horne, 2000, Smith, 1998). It is also an effective countermeasure against the post-lunch dip (Smith, 1998) or sleep inertia (Brice and Smith, 2002, Van Dongen et al., 2001). Plasma caffeine level peaks within 15–120 min after oral ingestion, and 99% of caffeine is absorbed from the gastrointestinal tract within approximately 45 min (Arnaud, 1998). Therefore, ingestion of caffeine followed by a short nap would be effective to reduce sleep inertia. Reyner and Horne (1997) reported that the combination of caffeine and a short nap was more effective to prevent sleepiness after a restricted prior nocturnal sleep, in comparison with caffeine intake alone or taking a short nap alone.

Bright light (>2000 lx) suppresses the secretion of melatonin and prevents the decline of body temperature during the night, so that it enhances alertness during the night shift (Badia et al., 1991, Campbell and Dawson, 1990, Daurat et al., 1993, Dawson and Campbell, 1991, Myers and Badia, 1993, Cajochen et al., 2000). During the daytime, when melatonin was scarcely secreted, the effects of bright light were hardly confirmed in some studies, in which nocturnal sleep was deprived or restricted (Badia et al., 1991, Daurat et al., 1993, Lafrance et al., 1998, Leproult et al., 2001). Other studies, in which prior nocturnal sleep was not restricted, however, reported positive effects of bright light during the daytime in psychological and psychophysiological measures (Grünberger et al., 1993, Saito et al., 1996). Therefore, if nocturnal sleep was not restricted, sudden bright light immediately after a short daytime nap may possibly act as an alerting stimulus to reduce sleep inertia.

In daily life, people often wash their faces to fully awaken from sleep. A few studies confirmed the alerting effects of face washing with cold water immediately after waking (reviewed by Ferrara and De Gennarro, 2000). It was also reported that cold air had marginal and transient effects for car drivers (Reyner and Horne, 1998). Therefore, it can be expected that cold stimuli to the face by water would act positively on sleep inertia after a short nap.

The aim of the present study was to examine the effects of caffeine, bright light and face washing on daytime sleepiness after daytime short naps. These conditions were compared with those of merely taking a nap and of not taking a nap.

In the present study, sleepiness was evaluated by subjective, behavioral and physiological measures (Curcio et al., 2001, Jewett et al., 1999). Subjective ratings of sleepiness and fatigue were measured using a 100 mm long visual analog scale. This is sensitive to sleep deprivation and to time of day (Babkoff et al., 1991). While becoming sleepy, response omissions increase and reaction time is prolonged (Doran et al., 2001). As a behavioral parameter, these performance decrease measures were used. As physiological measures, P3 amplitude of event related potentials (ERP) and EEG power spectra were used. P3 amplitude reflects attention allocation and memory updating, while its amplitude attenuates as a function of the reduction of alertness (Polich and Kok, 1995). Insertion of theta and alpha activities in the EEG recordings with eyes opened reflected a lowered alertness (Åkerstedt, 1991). Horne and Reyner, 1996, Reyner and Horne, 1997, Reyner and Horne, 1998, Reyner and Horne, 2000 used 4–11 Hz EEG activities as physiological sleepiness measures and confirmed the high validity.

Young adults participated in the present study. There may be several limitations in interpretation of their data. One reason is that individuals at this age have fewer complaints about sleep problems. However, afternoon sleepiness does occur similar to other older age groups, even if they had a longer nocturnal sleep than their usual sleep time (Carskadon, 1989). Therefore, their data could be applied for other age groups. Another problem is that sleep habits of this age group are prone to be irregular and to be shortened. Therefore, subjects were recruited who had regular sleep–wake habits, and had normal nocturnal sleep lengths.