variety of other factors. Yet with PSG and all of these other tools at our disposal, dreams have one remarkable feature that keep them out of reach of these tools: we can't report them in real time, while we're asleep. We can utilize any of our sophisticated tools to observe the sleeping person, but the only way to know what someone is dreaming is to wake them up and ask them. Imagine designing a method for measuring your experience of dreaming. With the least invasive tools we have, we could hypothetically let you fall asleep and then repeatedly awaken you, asking whether or not you were dreaming. Would you be able to remember, and how accurately? Would you be able to report the content of your dream?

All of this is not to say we can't study dreams at all, nor that we can learn nothing about them. Indeed, using all of the tools above and with repetition, we have been able to generate reasonable hypotheses. We do know that most dreams happen during rapid eye movement (REM) sleep, which is defined by rapid eye movements, and decreased muscle tone on the EEG, and that most people spend about two hours per night dreaming. To quantify the most elusive task of all, interpreting dream content, systems for quantitatively studying reported dream content have been developed by a team at UC Santa Cruz.2

There are clear links between psychiatric illness and dreams – indeed, nightmares following trauma are among core diagnostic criteria for PTSD (post-traumatic stress disorder), and we can even reduce the frequency of nightmares in people suffering from PTSD. Prazosin is not a sleeping pill per se, but rather an anti-nightmare medication that can tune down the fight-or-flight mechanisms that are active even in sleep with some forms of PTSD. 3

There are anecdotal and stereotyped dream experiences that are remarkably consistent across cultures and may guide us toward questions about innate learning and experience – dreams of falling, flying, being chased. Some dreams we associate in memory with anxiety – many have reported dreams of being in school or work naked, dreams of being underprepared. There are hobbyists and enthusiasts who attempt to hold on to self-awareness and experience so-called lucid dreaming, and some report being able to direct and remember their dreams. 4

We look to dreams for insight and for inspiration. They are a wellspring for narratives that enrich our human experience and they can guide us toward decisions we didn’t realize we were making. Problem solving, learning and consolidation of memories are proposed to be embedded in dreaming and sleep. The 19th Century German chemist August Kekulé, who discovered the ring structure of benzene, reported that he visualized the structure after dreaming of a snake eating its own tail. 5

So far, no adaptive function of dreaming has been demonstrated, yet history is rife with stories of revelatory dreams. Some dreams may be part of learning, some dreams may be reflective of our fears, some may help us learn about ourselves. There may be

several different dream states that we will one day parse into specific functions that operate discretely or simultaneously. For now, the meaning and function of dreaming lie just beyond our reach. 6

Much more amenable to study are the nuts and bolts of how we sleep, and what parts of our brains are active while we are sleeping. Falling asleep, we become drowsy, we drift through an invisible transition where we begin the cycles between the stages of sleep, and then we normally awaken equally seamlessly, ideally rested and ready to take on another day. We can observe sleep and the stages of sleep using the objective tools previously described. Our brains have a series of steps that we go through in that process of falling asleep. When the process of seamless transition from waking to sleep is misaligned and the steps don’t occur in the usual order, we can experience significant problems. There are a fascinating and well described (and fortunately treatable) set of sleep disorders called parasomnias, which occur during either REM sleep or NREM (non-REM) sleep. In NREM sleep, we see large amplitude slow waves on EEG.

NREM sleep patterns are distinct from REM sleep, with deeper stages of NREM sleep (also called delta sleep) having EEG waves that are slow and large in height. Seizures or epilepsy can present in similar fashion to many parasomnias, and so comprehensive evaluation is needed.

Some dreams may be part of learning, some dreams may be reflective of our fears, some may help us learn about ourselves.”

Shortly after falling asleep, and when we think most dreaming begins (about at the onset of REM sleep), there is a cellular switching mechanism located in the pons and midbrain (parts of the brainstem, the critical and very small part of the central nervous system between the brain and spinal cord), which stop you from moving, a state of relative atonia. You can still breathe, adjust your positioning and roll over (though some of those things happen during arousals when the switch is temporarily turned off, when you are partially but not completely awake), and your heart still beats as always, but volitional skeletal muscles won’t move. This stops you from acting out your dreams, potentially a very dangerous situation. 7

A few of the most common parasomnias involve problems in that switching process. If that internal switch mechanism is engaged too early while you are falling asleep, stopping you from moving volitionally, you can be awake and alert, but unable to move for a time until the rest of sleep is engaged. This REM parasomnia is called Sleep Paralysis, and the disorder associated is called Sleep Paralysis Disorder. Sleep paralysis can be addressed sometimes with medication but often does not require treatment and a regular sleep schedule with good sleep habits are all that's needed.

Another set of parasomnias involves a problematic lack of paralysis during sleep, where the atonic switch should be engaged, but is instead allowing us to move when we shouldn't, as seen in REM-sleep behavior disorder. In REM-sleep behaviors,