insomnia, characterized by fragmented REM sleep, leads to slow emotional processingDo you tend to dwell on upsetting thoughts? Does your arousal thermostat feel like it’s set too high, making it hard for your body to relax and fall or stay asleep? Insomnia is often described as a disorder of hyperarousal, yet how and why the hyperarousal develops is unclear.

Now 13 sleep scientists from three countries have taken findings from different lines of research, conducted a study of their own, and come up with a plausible explanation for why our minds and bodies feel like they’re stuck in overdrive and just can’t stop. Here’s what may lie behind our trouble falling and staying asleep.

Two Kinds of Sleep

Whether we sleep poorly or well, our nights are composed of two kinds of sleep: REM sleep, when most dreaming occurs and when the eyes move rapidly from side to side; and non-REM, or quiet, sleep. The deepest stage of non-REM sleep is called slow-wave sleep.

In the past, scientists thought that insomnia probably had to do with a dysfunction related to slow-wave sleep or some other feature of non-REM sleep. This may be the case for some people with insomnia.

But the results of two recent studies suggest that the problem may lie elsewhere. The findings show that sleep maintenance insomnia (the type of insomnia involving middle-of-the-night awakenings) is characterized by fragmented REM sleep.

Importance of REM Sleep

There’s solid evidence now that REM sleep—most of which occurs in the second half of the night—helps us process negative emotions such as fear and anger and self-conscious emotions such as guilt, embarrassment, humiliation, and shame. In studies where participants are forced to rise too early, they often wake up in a bad mood.

The reason is that they’ve gotten insufficient REM sleep, say sleep scientists, and the full processing of emotion has not had a chance to occur. As a result, any negative emotion they experienced the previous day may still retain its emotional charge.

There’s also preliminary evidence that fragmented REM sleep (such as that found in insomnia) hinders the overnight resolution of emotional distress.

Authors of the current study, published in Proceedings of the National Academy of Sciences, wanted to take this research further. They predicted that fragmented (or restless) REM sleep could interfere with the overnight resolution of emotional distress, thus contributing to the accumulation of arousal, and eventually hyperarousal, associated with insomnia.

A Two-Part Study

First, the researchers tested 32 people—16 with insomnia disorder and 16 normal sleepers—in a sleep lab for two nights. The object was to find out if restless REM sleep correlated with the frequency of participants experiencing thought-like rather than dream-like mental activity at night (as assessed by questionnaires). Thought-like nocturnal mentation—when we’re mentally chewing on a problem, for example—is specifically associated with restless REM sleep.

In addition, the researchers looked for relationships between restless REM sleep, duration of emotional distress, and chronic hyperarousal in data compiled on 1,199 people participating in the Netherlands Sleep Registry. The NSR is a psychometric database created to facilitate research on factors that discriminate people with insomnia from people without sleep complaints.

Restless REM Sleep, Emotional Distress, Hyperarousal

The investigators used sophisticated statistical methods to analyze their data, and here’s what they found:

  • Compared with normal sleepers, insomniacs were slower to recover from long-lasting emotional distress. The more severe the insomnia, the slower the recovery.
  • Thought-like mental activity was more frequent among insomniacs than normal sleepers, and it was significantly associated with emotional distress lasting overnight (but not with short-lasting distress resolved during the previous day).
  • The more severe the insomnia, the more people reported symptoms of hyperarousal.
  • Long-lasting emotional distress accounted for 62.4% of the association between hyperarousal and the thought-like mental activity occurring during restless REM sleep.

What It Means for Us

The findings conformed to researchers’ expectations. So here’s their explanation for why insomniacs, both mentally and physiologically, keep going and going and can’t seem to relax into sound, consolidated sleep: mental activity we experience at night—which is associated with fragmented REM sleep (and increased eye movement density)—interferes with the overnight resolution of emotional distress. So we awaken with the distress still in place, which ramps up arousal. Over time, this could lead to a state of chronic hyperarousal.

It sounds plausible to me. Your thoughts?

Posted by Lois Maharg, The Savvy Insomniac

Lois Maharg has worked with language for many years. She taught ESL, coauthored two textbooks, and then became a reporter, writing about health, education, government, Latino affairs, and food. Her lifelong struggle with insomnia and interest in investigative reporting motivated her to write a book, The Savvy Insomniac: A Personal Journey through Science to Better Sleep. She now freelances as an editor and copy writer at On the Mark Editing.


  1. It certainly sounds plausible to me, too. Now, how to get off the merry go round?



    1. I’m with Nancy. It’s just what I’d expect. Now, what can we do about it? Thank you for your clear explanation.



    2. Hello Nancy & Eric,

      At the end of their paper, these authors suggest “targeted interventions to either restore consolidated REM sleep or, if not feasible, prevent the occurrence of fragmented REM sleep.”

      Are there any medications out there that would restore consolidated REM sleep? I really don’t know.

      But I know sleep scheduling has helped make my sleep less fragmented. For years the idea was totally unappealing, but since going through sleep restriction, I’ve become a believer. Sticking to regular bed and rise times, I don’t wake up at night the way I used to.

      The other thing that helps me consolidate my sleep is daily exercise late in the afternoon, although I suspect the real benefit I get from this is being able to fall asleep more quickly at the beginning of the night.



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