Sleep Lab, Department of
Psychology, University of Florence,
Italy
Yawning occurs more frequently in the early
morning and in the late evening, close to sleep
onset and after the awakening, and it might be
linked to sleep propensity. We aimed to study
yawning and its temporal distribution in morning
and evening subjects who display different
sleep-wake and sleepiness rhythms.
Sixteen healthy young adults (8
evening-types and 8 morning-types, matched for
age and gender) have been selected and
instructed to keep their habitual sleep
schedules and to signal every yawning occurrence
for three consecutive days.
Results show that evening-types yawn more
frequently than morning-types, particularly
during morning hours. Yawning frequency
decreases across daytime in evening-types
reaching its lowest level in the early evening
and increases thereafter. Instead, in
morning-types, yawning frequency remains quite
low during daytime and increases in the evening.
Moreover, both morning and evening types show a
progressive increase of yawning frequency in the
hours preceding sleep onset, whereas they differ
after the awakening.
Evening-types show a higher yawning
frequency that remains quite stable in the hours
following the awakening, while morning-types
display a decline in yawning frequency. Our
findings show that the temporal distribution of
yawning frequency differs between chronotypes,
supporting the hypothesis that differences in
sleep-wake rhythm affect yawning, which could
represent a behavioural sign of sleep
propensity.
1. Introduction Yawning is an involuntary
and stereotyped behavioural event [1,2].
Its frequency increases in the early morning and
in the late evening [3] and it is
strongly associated with periods preceding the
sleep onset and following the awakening
[3&endash;5], when the sleepiness level
is particularly high. Moreover, several studies
showed that yawning consistently precedes
motility level increase [3] and
electroencephalographic activation
[6].
According to these results, yawning is
supposed to activate the organism in order to
counteract the increase of sleepiness
[1]. This implies that yawning would
occur more frequently in those situations when a
high level of arousal is needed [7].
Chronobiological studies have described two
extreme chronotypes: on one hand, people who
prefer to get up early in the morning and report
difficulties in keeping themselves awake beyond
their habitual bedtime (morning-types); on the
other hand, people who prefer to go to bed late
at night and claim difficulty in morning
awakening (evening-types) [8]. Compared
with morning-types, evening-types show delayed
circadian rhythms, including sleep&endash;wake
pattern [8&endash;11], core body
temperature [8&endash;10,12&endash;14]
and sleepiness [9,14,15]. Moreover,
evening-types rated themselves sleepier than
morning- types during morning hours
[14,16]. The phase&endash;relationship
between sleep and core body temperature is
affected by chronotypology as well
[11,13].
Awakening in evening-types is closer to
temperature nadir than in morning-types.
According to Baher et al. [13],
evening-types sleep on an earlier part of their
temperature cycle than morningtypes and, as a
consequence, they are more alert at bedtime and
sleepier after awakening. More recently,
differences between chronotypes in the
homeostatic component of sleep regulation have
been also pointed out: the kinetic of sleep
pressure, concerning both rise [14] and
dissipation rate [17,18], is slower in
evening-types than in morning-types.
Taking into account the close relationship
between yawning and sleep propensity, we
hypothesize that differences in sleep&endash;
wake and sleepiness rhythms between chronotypes
could affect yawning distribution across
24-hours and its relationship with sleep onset
and awakening. In particular, we expect a higher
yawning frequency in evening-types than in
morning-types during the morning and
particularly in the hours following the
awakening.
4. Discussion Consistently with
previous researches [8,9,19],
eveningtypes show a delayed sleep&endash;wake
rhythm compared to morning-types. Moreover, the
groups do not significantly differ in sleep
duration and they show almost identical sleep
latency and efficiency. Daily yawning frequency
tends to be higher in evening-types than in
morning-types, and it is not correlated with
sleep duration. The group difference in total
daily yawns closely approaches statistical
significance, notwithstanding the small number
of subjects and the high inter-individual
variability showed by both groups. A great
disparity in yawning frequency has been
previously reported [3,20], suggesting
that yawning itself is a phenomenon
characterized by large differences in frequency
between individuals.
The evening-types' general tendency to yawn
more frequently than morning-types is mainly due
to yawning that occurs in the morning. Actually,
evening-types yawn more frequently than
morning-types from 10:00 to 12:00. This finding
is in agreement with results which consider
yawning a sign of sleepiness [4] and
which show that, compared to morning-types,
evening-types display poorer alertness at
wake-up time [21] and higher sleepiness
in the morning [14,16]. Our results show
that yawning frequency varies according to
chronotype and the time-of-day. It decreases
across daytime in evening-types, reaching its
lowest level in the early evening, and increases
thereafter. Whereas, yawning frequency in
morning-types remains quite low during daytime
and increases in the evening. As expected, the
distributions shown by the two chronotypes
parallel the subjective sleepiness time course
found by previous study [14], and
display a mirror-like fluctuation with respect
to subjective alertness distribution
[16]. Actually, evening-types yawn more
frequently than morning-types during morning
hours, just when the former commonly rate
themselves sleepier or less alert than the
latter.
With regard to the relationship between
yawning and sleep onset, both morning and
evening types show a progressive increase of
yawning frequency in the hours preceding sleep
episode, without differences between the two
groups in the amount of yawns. This rise
confirms the relationship between yawning and
sleep onset previously reported in non-selected
subjects [3&endash;5]. Furthermore, the
absence of group differences could be explained
assuming that both morning and evening types
went to sleep when their sleep propensity was
prominent.
This hypothesis is supported by the fact
that habitual bedtimes reported by our subjects,
both morning and evening types, are fairly close
to their preferred ones. On the other hand,
chronotypes show a different behaviour after the
awakening. Evening-types show a higher yawning
frequency that remains quite stable in the hours
that follow awakening, while morning-types
display a yawning frequency decline. This
finding suggests the permanence of daytime
sleepiness in evening-types, or their inability
to fully wakeup. Consistently, evening-types
rate themselves sleepier than morning-types
during morning hours [14].
The higher sleep propensity and the
consequent lack of yawns reduction after the
awakening in the evening-types could be
explained by an advanced get-up time due to
social constraints. The wide gap between
preferred and actual get-up time observed in the
evening-types substantiates this hypothesis. Our
results strengthen the relationship between
yawning and sleepiness: yawning is more likely
to occur in that part of day when extreme
typologies generally claim detrimental
sleepiness level and it shows a different trend
after awakening according to chronotype sleep
propensity differences.
-Giganti F, Hayes
MJ Cioni G, Salzarulo P Yawning frequency
and distribution in preterm and near term
infants assessed throughout 24-h recordings
Infant Behav & Development
2007;30(4):641-647
