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EFFECTS ON SLEEP 49
the laboratory. Brink, Müller and Schierz (2006) describe a more sophisticated
method which is based on the bed being placed on accelerometers. This allows the
tracking of whole body movements.
Motility is related to many variables of sleep and health (Reyner, 1995; Reyner et al.,
1995; Passchier-Vermeer et al., 2002). Clinical research shows that the sleep/wake
cycle (assessed by polysomnography, EEG, EOG, EMG) passes through the 24-hour
period synchronously with the rest/activity cycle (assessed by actimetry) (Borbely et
al., 1981). A number of investigations have compared the results of polysomnographic
recordings (number of EEG-awakenings during sleep period, duration of
sleep period, sleep onset time, wake-up time) with results of actimetry. The correlation
between actimetrically assessed duration of sleep period, sleep onset time, wakeup
time and similar variables assessed with polysomnography was found to be very
high (correlation coefficients between individual test results in the order of 0.8–0.9).
Measures of instantaneous motility are the probability of motility and the probability
of onset of motility in a fixed time interval, for example a 15-, 30- or 60-second
interval. Increased instantaneous motility during sleep is considered to be a sensitive
behavioural marker of arousal, but the relation with arousal is not simple. Also
other factors, such as the need to relieve the pressure on body parts for better blood
circulation, cause motility, and spontaneously occurring arousals are part of the normal
sleep process. The noise-induced probability of (onset of) motility is the difference
between the probability of (onset of) motility during noise events minus the
probability in the absence of noise.
Onset of motility and minor arousal found on the basis of EEG recordings are highly
correlated. In the United Kingdom sleep disturbance study, Ollerhead et al. (1992)
found for their study population that during sleep there is on average an EEG
(minor) arousal in 40% of the 30-second intervals with onset of motility.
Unfortunately, it is unknown whether this 40% is also valid for noise-induced awakenings.
In 12% of the 30-second intervals with an EEG (minor) arousal, motility
does not occur. Several field studies (Horne et al., 1994; Fidell et al., 1998, 2000;
Flindell, Bullmore and Robertson, 2000; Griefahn et al., 2000; Passchier-Vermeer et
al., 2002, 2004) have been conducted regarding noise-induced instantaneous motility.
For this effect, relationships have been established with SEL or L Amax , for aircraft
noise only. In Passchier-Vermeer et al. (2002) relationships between noiseinduced
increase in motility or noise-induced increase in onset of motility in the 15-
second interval with the maximum noise level of an overflight, and L Amax or SEL
have been approximated by quadratic functions (see, for instance, Fig. 3.2). It may
be noted that the threshold of motility (L Amax = 32 dB(A)) is in the same range as
the threshold found by Basner et al. (2004) for EEG awakenings, with a definition
that also encompassed transitions to steep stage 1 (L Amax = 35 dB(A)). The probability
of motility at 70 dB(A) of about 0.07 is lower than the probability of noiseinduced
EEG awakening at L Amax = 73 dB(A) of about 0.10. There is no a priori
reason to expect the above threshold probabilities to be the same for these two
measures of sleep disturbance, but, taking into account that motility is assessed for
shorter intervals (15 seconds vs. 90 seconds), the differences in probabilities above
threshold appear to be limited.
One of the variables influencing the relationships between noise-induced instantaneous
motility and L Amax or SEL, is long-term aircraft noise exposure during sleep.
The probability of instantaneous aircraft noise-induced motility is lower when the
long-term exposure is higher. This may be partly due to the higher base rate motility
in quiet intervals in higher long-term exposure, which is used as a reference for
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