Chapter
Fifteen
Dreaming and beyond
both of which disrupt our ordinary sense of being a conscious self who looks out
from inside our skin.
WAKING AND SLEEPING
I’ve dreamt in my life dreams that have stayed with me ever after,
and changed my ideas: they’ve gone through and through me, like
wine through water, and altered the colour of my mind. And this is
one: I’m going to tell it – but take care not to smile at any part of it.
(Emily Brontë, Wuthering Heights, 1847)Every day we all go through a cycle of three states: waking, REM (rapid eye
movement) sleep, and non-REM sleep, a typical night’s sleep consisting of four
or five cycles between non-REM and REM sleep, and often some unremembered
micro-awakenings. These waking and sleep states are defined by physiological
and behavioural measures, including how easily the person can be awakened,
their eye movements and muscle tone (the degree of passive contraction in the
muscle fibres), and their brain activity as measured by either EEG or scans. In REM
sleep, the brain is highly active and the EEG resembles that of waking, although
paradoxically, the sleeper is harder to wake up than during non-REM sleep. Even
in non-REM sleep, the overall firing rate of neurons is as high as in waking states,
but the pattern is quite different, with the EEG dominated by long, slow waves
rather than complex, fast ones.
The neurochemistry and physiology of these states is well researched. For exam-
ple, the neuromodulators adenosine and melatonin play crucial roles in inducing
sleep. During sleep, the REM cycle is controlled by the reticular formation in the
pons in the brainstem, and not by higher brain areas, which are unnecessary for
normal sleep cycling. Within the brainstem are cholinergic REM-on nuclei and
aminergic (both noradrenaline and serotonin) REM-off nuclei, which reciprocally
activate and inhibit each other and control the switching of states.
During sleep, parts of the brain are isolated in different ways and to different
extents. Blocking of sensory input happens at the thalamocortical level in non-
REM sleep and at the periphery in REM sleep. There are also different phases of
REM, and fMRI studies show that during tonic (persistent) REM auditory stimuli
still activate auditory cortex to some extent, while during phasic (intermittent)
REM, when eye movements and muscle twitches occur, the brain operates in a
functionally isolated closed loop (Wehrle et al., 2007).
In REM sleep, the brain stem blocks motor commands at the level of spinal motor
neurons so that whatever is going on in motor cortex does not result in physical
activity. This means you can dream of climbing out of the window onto the roof,
but your legs won’t let you do it – although these protective mechanisms can break
down briefly in sleepwalking, and are overactive in sleep paralysis. At the same time,
the pons, amygdala, hippocampus, and anterior cingulate are especially active, as are
parts of the visual system and visual association areas, but the dorsolateral prefrontal
cortex (associated with executive functions like working memory, problem-solving,
and planning, as well as motor organisation) is much less active than during waking.