No, parents, you aren’t crazy. Interrupted sleep can tank your mood and impair your functioning — above and beyond the harm caused by getting less sleep overall. But why do sleep disruptions have these effects? Research tells us why we should take the problem seriously, and it offers helpful insights. The important thing is to protect your brain’s ability to get enough slow-wave sleep. If you can lock down at least one, 3-4 session of uninterrupted sleep — and take other measures to improve sleep quality — you can improve your health and well-being. Poor sleep is a major complaint for parents. How bad is it? If you have a child who keeps waking you up at night, you don’t need me to tell you how you feel. You’re exhausted, strung-out, or lost in a zombie-like haze. The first weeks of infant care can be especially tough. Newborns need frequent feedings, and their sleep patterns are out-of-sync with the natural rhythms of day and night. As babies mature, they develop more mature sleep patterns, which is helpful to us. But night wakings continue for months. And even when children are older, they may experience sleep disorders that disrupts our sleep routines. According to a study conducted in Germany, parents may not re-establish satisfactory sleep patterns until several years after the birth of their first child. It’s one of more obvious burdens that parents struggle with. Yet when researchers have tried to quantify it — by measuring sleep in new parents — they have sometimes reported numbers that seem too good to be true. The sleep loss that (on paper) sounds relatively mild. How much sleep do new parents typically get? Researchers may come up with somewhat different estimates depending on the methods they use. They could simply ask parents to provide their own estimates (e.g., “how much sleep are you getting these days?”), which is the approach taken by many non-scholarly, popular surveys. But this isn’t as accurate as either (1) asking parents to keep a detailed sleep diary for several days in a row, or (2) asking parents to wear movement-detecting devices (wrist actigraphs) that measure sleep-time in a relatively objective way. In some studies, researchers do both. What do these more accurate methods reveal? Numbers that have surprised me. For example, during the first 8 weeks after childbirth, mothers are often reported as getting (on average) at least 7 hours of sleep. And when researchers have tracked changes in sleep duration from pregnancy to new parenthood, they’ve noted reductions that are important, but not super-dramatic. In a recent meta-analysis of some of the best studies available, the average amount of sleep loss experienced by new parents each night — throughout the first 16 weeks postpartum — was estimated at just 43 minutes. Of course, these estimates come with a lot of cautions. During the later months of pregnancy, many women experience sleep problems, which can include short sleep duration. So if new mothers are losing an average of 43 minutes of sleep after the baby comes, that may represent an increase over and above whatever sleep deficit they were already coping with. Moreover, any talk of averages alone can’t help us appreciate the range of variation. Many folks are coping with higher-than-average levels of sleep deprivation. Still, my mind boggles when I read results like these. How do we reconcile such reports with our everyday experiences? The research makes it sound like baby-care isn’t really such a sleep-sapping proposition after all. Not for most people. And that can’t be right. But here’s the thing… Total sleep time isn’t the only aspect of sleep that goes haywire when you become a parent. How often are you being awakened each night, and how disruptive are these wakings? How much time are you spending awake after initially falling asleep (what investigators term “Wake After Sleep Onset”, or WASO)? When researchers focus on sleep interruptions, rather than total sleep time, they get a very different picture of how sleep changes after a baby. For instance, that 43 minute average sleep deficit for parents during the first 16 weeks postpartum? It represents a reduction of sleep time by approximately 10%. By contrast, researchers found that average WASO increased by 47% during the same period. So it’s likely that interrupted sleep has something to do with the strung-out, zombie-like feelings that we experience during postpartum life. But why should this be? Understanding sleep cycles: Why interrupted sleep causes trouble If something keeps waking you up at night, it’s pretty obvious that you’re at higher risk for getting less sleep overall. You might have trouble falling back to sleep, so you aren’t able to take full advantage of the time you have left. In this way, interrupted sleep can lead to restricted sleep — less total sleep time. But there is another way in which fragmented sleep can cause problems, and to understand it, you need to think about human sleep cycles. When we’re allowed to doze normally, sleep unfolds in a series of stages, beginning with two stages of light sleep (NREM1 and NREM2); continuing with a stage that emphasizes, deep, slow-wave sleep (NREM3); and ending with rapid eye movement sleep (REM). A single cycle takes 90-110 minutes to complete, on average, at which point we either awaken, or continue with another cycle. And adults usually complete 4-6 cycles each night. But the crucial point is this: All sleep stages are NOT equally restorative or crucial for our health. In experiments where people have been deprived of all sleep, it appears that NREM3 is the stage that the brain misses the most. When study volunteers are finally permitted to snooze, their brains prioritize slow-wave sleep. Moreover, the extent to which people bounce back after sleep deprivation — the degree to which they become alert, focused, and competent again — depends on the intensity of their deep, “recovery” sleep. In fact, researchers have been able to minimize the cognitive side effects of sleep deprivation (such as attention lapses, and working memory performance) by artificially enhancing slow-wave activity during sleep recovery. In addition, there is strong evidence that NREM3 helps us maintain a strong, healthy immune system, and that deep, slow-wave sleep promotes growth, repair, learning, hormonal regulation, and the removal of hazardous protein clumps (amyloid plaques) that build up in the brain when we’re awake. So slow-wave sleep plays a crucial role in your well-being What happens if someone wakens you when your brain is in NREM3? Or awakens you even earlier in the sleep cycle – before you’ve even had the chance to transition into NREM3? The opportunity for restorative, deep sleep gets interrupted, and when you finally get the chance to resume sleeping, you probably won’t get to pick up where you left off. Instead, you’ll start over again with light sleep. And if keeps happening, you might end up feeling pretty wretched in the morning – and worse than might be expected based on the sheer amount of time that you spent sleeping. This, at any rate, is a favored theory. How does it stack up against the evidence? Experimental evidence: Interrupted sleep worsens mood and frazzles attention In one of my favorite sleep experiments, Michal Kahn and her colleagues at the University of Tel Aviv recruited 61 young adults (healthy college students free of baby-related responsibilities) and asked them to wear wrist actographs at night. Actigraphs detect movement, providing researchers with an objective way to estimate sleep time. Kahn’s team measured sleep on a normal night, and checked participants’ psychological responses the following day — measuring their moods, as well as their ability to stay focused during a demanding attention task. In addition, the researchers randomly-assigned each participant to one of two treatment conditions: • a night of sleep restriction (permitted to sleep no more than 4 hours at night); or• a night of induced wakings (forced to awaken 4 times, over the course of 8 hours). People…