Left-Handedness, Brain Evolution, Sleep, and Dreams

We are one of the few species that exhibits population-wide consistent hand-preferences or handedness (that is most of us are right-handed). Handedness, therefore, indexes something unique and special about the human brain. While that specialness certainly points to the left-hemispheric preference for speech and language functions, sleep may also have played a role in the evolution of brain lateralization.

About 10-12% of the human population is left-handed. Nevertheless, language representation among lefties is still predominantly left-hemisphere based. Neuroimaging and behavioral studies suggest language is lateralized to left hemisphere in roughly 80% of left-handers. In contrast, it is lateralized to the left hemisphere in about 95% of right-handers. In short, key language functions are lateralized to the left in most human beings regardless of handedness. It may be that some other behavioral function besides language is influencing handedness.

In the last couple of decades, it has become clear that left-handers consistently report poorer quality sleep, decreased time spent in non-REM (NREM) sleep, increased time spent in REM sleep, (but fewer REM episodes per night), and increased interhemispheric EEG coherence during sleep (there is greater cross-talk between the two hemispheres). Left-handers tend to exhibit greater REM durations and greater bilateral brain activity during REM than right-handers.

Why should left-handers exhibit this profile of increased REM duration and increased bihemispheric cross-talk traffic during REM?

First of all, it should be recalled that for most species NREM sleep is unihemispheric; that is it occurs in one hemisphere at a time. The fact that NREM requires only one hemisphere for its expression raises the question of whether that is the case for REM as well. To my knowledge REM can only be expressed bihemispherically. It appears to require two hemispheres to be fully expressed. I do not claim that that is established scientific fact. It is simply my impression of the facts after reading the literature on EEG – based sleep studies in birds and mammals. REM appears to require larger parts of the brain than NREM to be expressed. That does not mean that NREM never recruits sites in both hemispheres of the brain. It does. It simply means that slow wave delta activity can occur locally while REM-related de-synchronization typically involves inter-hemispheric α and β synchrony (which resembles α and β synchronization of resting wakefulness). Imbach et al (2012) documented robust interhemispheric oscillation during rapid eye movement (REM) sleep in humans and Nielsen et al. (1990) reported increased EEG coherence in left-handed versus right-handed individuals during wake, stage 2, and rapid eye movement (REM) sleep.

Given the fact that REM sleep involves increased hemispheric communication relative to NREM, Christman and Propper (2010) suggested that individuals who tend to have greater overall interhemispheric communication (e.g. left-handers) should express REM more fully relative to more lateralized individuals (e.g. right-handers). We have already seen that lefties do, in fact, exhibit longer REM durations but fewer REM periods. With greater amounts of REM and interhemispheric crosstalk during REM do lefties also report more intense dreams? McNamara, Clark, and Hartmann (1998) found that the dreams of left-handers contained more high imagery nouns, more affective words, and were more fictional. Hicks and colleagues (1999) found that left-handers had significantly more lucid dreams and fantastic nightmares than right-handers. Paradoxically, however, lefties seem to recall fewer dreams than right-handers but this may be due to the fewer REM episodes they experience.

So what does all of this mean? REM is bilaterally expressed in both right and left-handers but because lefties are less lateralized than right-handers, REM may be expressed more intensely in lefties resulting in poorer sleep quality (due to less NREM), more vivid dreams and some nightmares. But greater intensity of REM in lefties may also be a source of their greater performance variability on intelligence and creativity tests in many domains of human activity. That is because REM may be an engine that promotes brain development and greater brain processing capacities.

It may be that REM evolved in order to increase brain processing capacity. In a phylogenetic comparison of human vs non-human primate sleep quotas, Samson and Nunn (2015) discovered that relative to other primates, humans have exceptionally shorter sleep times but a significantly higher proportion of REM, suggesting that we opted to increase sleep intensity in the form of REM quotas to support longer brain development times and greater, more bilateral processing capacities. If greater amounts of REM brings some woe to lefties in the form of poorer sleep quality, it may also be a source of greater variability in brain functioning and thus the greater creativity found in some lefties as well.