FoundationalConceptsNeuroscience

rectable by simply getting a good night’s sleep. Are there other longer-
term effects of not getting enough sleep, perhaps things that are not
so readily reversible? Again, why do we need to sleep?
One hypothesis is that during sleep various restorative processes
operate within the body. The chemical reactions of cellular metabo-
lism—taking place continuously, maintaining life in each and every
cell of our body—can have damaging consequences. Highly reactive
molecular entities, such as free radicals of various kinds and excited
electronic states of oxygen, are constantly being generated. These
molecules have detrimental effects on their surroundings, reacting
with cellular components in ways that are damaging. It is absolutely
essential for all living cells to have mechanisms for neutralizing free
radicals and oxidants and for repairing molecular damage (such as to
DNA), in order to sustain life. It may be that during periods of sleep
and torpor some of these protective and repair processes can more
effectively carry out their tasks.
One consistent finding over the last couple decades of research is
that sleep plays a critical role in learning and memory. First, sleep
somehow prepares the brain to optimally encode new information for
storage in long-term memory, and sleep deprivation dramatically im-
pairs this process. A good night’s sleep is essential to optimize learning
the next day. After the initial encoding of a memory in the neural
circuitry of the cortex, an extended period of time is necessary to lock
the memory more securely in place—the process of consolidation (see
Chapter 19). Substantial aspects of memory consolidation appear to
take place during sleep. This includes reactivation of the networks
involved in the initial experience of whatever is being learned. Studies
of both rats and humans have shown that configurations of brain
neurons active during the learning of a task become active again dur-
ing sleep. It is likely this repetition of activity is part of a process of