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EFFECTS ON SLEEP
ing sleep stages 1 and 2) increases with age (Mathur and Douglas, 1995; Boselli et
al., 1998) possibly only for men (Hume, Van and Watson, 2003), their average length
is stable and circa 15 seconds (Boselli et al., 1998). Also the threshold for auditory
arousal decreases with age (Zepelin, McDonald and Zammit, 1984; Busby and Pivik,
1985; Busby, Mercier and Pivik, 1994) and towards the end of the night (Basner et
al., 2004). Recovery after EEG awakenings takes longer for noise-induced awakenings
than for spontaneous awakenings (Basner et al., 2004). The time required for
falling asleep again depends on the sound level and especially for loud events this
latency is considerably longer than after spontaneous awakenings. Thus, in general,
noise-induced EEG awakenings are more disruptive than spontaneous awakenings
and therefore will more often be experienced consciously and remembered afterwards.
In common situations with aircraft overflight noise at home, (minor) arousals
were found in 10.3% of the 64-second intervals without aircraft noise and this percentage
was found to be increased by circa 4% up to 14.3% in intervals with an aircraft
noise event (Hume, Van and Watson, 2003). Thus, in that particular (exposure)
situation, about 1 in 24 aircraft overflights caused a (minor) arousal.
3.1.3 MECHANISMS
Activity in the auditory system up to the brainstem nucleus inferior colliculus occurs
within 10 milliseconds after the onset of a sound. This early activity appears to be
obligatory and is hardly affected by the state (sleeping or awake). Being asleep or
awake does influence later activity. The auditory pathways proceed from the inferior
colliculus to the thalamus and from there to the auditory cortex. The state (asleep
or awake) affects the activity in the thalamocortical circuits, which occurs after
10–80 milliseconds. In particular, during SWS the transmission of auditory information
through the thalamus is suppressed. This is not the case during REM sleep or
when awake.
Thus, while in all (sleep) stages, sound activates the auditory system up to the inferior
colliculus, the sound-induced activation of higher areas is suppressed in SWS.
Therefore, further activation depending on those higher areas (for example, extracting
meaning) is not likely to occur as a primary reaction to sound during SWS. For
understanding arousal during SWS, it is important that the inferior colliculus and the
earlier auditory nucleus of the lateral lemniscus, and also the (dorsal and ventral)
cochlear nuclei project to reticular arousal system. Presumably, sound is always
capable of arousing the sleeping subject through these connections. The ascending
arousal system is heterogeneous and encompasses mono-aminergic, glutamatergic,
and cholinergic nuclei that can directly or indirectly activate the thalamus and cortex.
An important indirect route is the activation of the basal forebrain, which can
activate the cerebral cortex through widespread, mainly cholinergic projections. The
activation of the thalamus and cortex is indicated by an increase in EEG rhythm frequency
and a reduction of the inhibition in the thalamic sensory relay nuclei. As a
consequence of the latter effect, subsequent sound-induced activation may pass the
thalamus and may be subject to more elaborate processing than initial sound. It can
be speculated that sound in this way also reduces the threshold for somatosensory
information that initiates body movements so that more body movements are
observed when exposed to sound. The occurrence of habituation of cortical responses
suggests an active role played by a part of the brain that blocks or at least limits
the impact of the activated ascending pathways.
NIGHT NOISE GUIDELINES FOR EUROPE