This scientific article was written by Jessica Page, a post-doctoral student at Northwestern University’s Feinberg School of Medicine who researches childhood development.
At some point or another, we accept that aging is a part of life. We easily notice how getting older affects our behavior, learning, and memory. But many of us overlook the impacts of aging on sleep and, conversely, the impacts of sleep on aging. These two elements of life are deeply intertwined. Let’s explore why this is and discover the biological changes that underpin sleep.
What Determines Our Need to Sleep?
Before we address the role of aging and sleep, we need to first understand what is sleep and what determines how much we sleep. Sleep is not a passive event, but rather an active process involving physiological changes that occur throughout the brain and body (Guidozzi, 2015). Sleep is governed by two processes (Borbély, 1982): one oversees the time of day to sleep during a 24-hour period (Daan et al., 1984) and the other gauges the need for sleep (Achermann & Borbély, 2011). Your need for sleep is determined by circadian rhythms and the interaction of these processes defines the timing and duration of your sleep. Throughout the day or with extended wakefulness, our sleep pressure increases and thus, the need for sleep increases (Dijk & Franken, 2005). Yet, many factors such as genetics, feeding, exercise, stress, menstrual cycle, hormones, and medications also influence our sleep need.
What Is Sleep Composed Of?
Different factors affect specific parts of sleep. Our internal clock, which is present in cells and neurons in the body, is influenced by light-dark cycles as well as eating (Santhi et al. 2016). This clock plays an important role in the sleep/wake cycle and enables the transition of different sleep cycles (Guidozzi, 2015). Circadian rhythms influence not only our sleep but also alertness, mood, hormone release, all of which are controlled by our internal clock (Achermann & Borbély, 2011).
When we fall asleep, we experience two alternating stages, Non-rapid eye movement (NREM) and rapid eye movement (REM) sleep (Iber et al., 2007; Rechtschaffen & Kales, 1968). NREM sleep is further divided into sleep onset, light sleep, and slow wave sleep (SWS) – also called deep sleep (Iber et al., 2007). REM sleep is well-known for periods in which dreaming occurs, as well as bouts of rapid eye movement and lack of muscle tone or strength (Iber et al., 2007). Over the course of a typical sleep cycle, humans alternate between periods of NREM and REM sleep, lasting between 90 to 120 minutes per stage (Feinberg & Floyd, 1979). Thus, the average adult experiences 4 to 6 NREM-REM sleep cycles. Depending on how long one sleeps, throughout the night with each consecutive cycle, there is less NREM sleep and more REM sleep (Haggenauer et al 2006; Dement and Kleitman, 1957).
Measuring Sleep: How Do I know If I’m Getting “Good Sleep”?
Sleep health is determined by the amount of time it takes to fall asleep, sleep duration, staying asleep (not waking up at night), and consistencies in your sleep routine (Carskadon et al, 2011). While these factors seem straightforward, it can be challenging to know if you are getting “good sleep” and what to do if you’re not. It’s also often difficult to determine how your sleep needs may change with age. Change in sleep is a part of aging and aging impacts your sleep health. With age, our sleep architecture (or sleep patterns) also change. As a result, our circadian rhythm changes, which then alters our sleep cycles and ultimately our sleep health. Throughout the night we experience a repetition of sleep cycles and though the total amount of sleep generally remains the same, with age we spend more time in lighter stages of NREM sleep and may not experience as much REM or deep sleep (Carskadon et al, 2011). Understanding some of these changes may help you recognize if alterations in sleep quality are due to aging and identify if these changes are a concern.
Sleep for Early Childhood
Sleep patterns change across the lifespan, but the most change is seen in early childhood. Early childhood is characterized by rapid development in brain development and learning (both of which are thought to be supported by sleep). Many people with newborns may feel like their child’s sleep need changes from day-to-day, and rightly so. Newborns don’t have an established circadian rhythm and, as a result, spend upwards to 16 hours of their day sleeping. And, since they have shorter sleep cycles their sleep is spread throughout the day (Davis, Parker, & Montgomery, 2004). During early childhood, there’s a decline in sleep from 14-16 hours and 2-3 naps in newborns to 10-12 hours and 1 nap in preschool-aged children. Still, we sleep longer in early childhood than any other period of life. This is the case for a couple of reasons: First, our brains are plastic, changing at impressive rates, and this change seems to parallel the changes in our sleep. Second, when we are born our brains are not fully developed and sleep helps our brains restore information that was learned throughout the day (Kurth, Achermann, Rusterholz, & Lebourgeois, 2013).
As children grow older they experience a decrease in REM sleep and an increase in NREM sleep. The change is thought to mirror aging and cortical development (Kurth et al., 2010), social-emotional development (Mindell, Leichman, DuMond & Sadeh, 2016), and language and cognitive development (Page, Lustenberger, & Frohlich, 2018). As children age and spend less time sleeping, they have more opportunities to play and learn new information. The newly learned information is then strengthened and stored during sleep. Children who do not acquire an adequate amount of sleep or deprived of their daily nap, are not only irritable but show decreases in performance (Kurdziel et al. ,2013) and accuracy (Astill et al., 2014) compared to children who receive an adequate amount of sleep. By the time children begin elementary school, they typically have a set sleep schedule where all of their sleep occurs at night.
Sleep for Adolescence
The transition to adolescence is a period that many of us remember and some may wish to forget. Adolescence can be described as a time of social, emotional, and cognitive transition from childhood to adulthood (Haggenhouer et al 2006) and may be best known as the moment of puberty. This period of life takes place when we’re 10-19-years old, and like early childhood, it’s another time of vast changes in sleep patterns. It’s during this moment in life when gender differences in aging and sleep become more apparent. Girls have a longer sleep duration more deep sleep, fewer wakings and better sleep efficiency than boys (Mehta, Shafi, & Bhat, 2015).
Due to changes in reproductive development and changes in melatonin, a hormone that alters sleep, adolescents experience many changes in their sleep-wake cycle. During this age span, there is reduced deep sleep (N3) and REM sleep as well as an increase in delayed sleep phase. (Jenni and Carskadon, 2004; Jenni et al., 2005; Kurth et al., 2010; Lui et al., 2017). In other words, adolescents are going to bed later and not getting enough of the deep sleep they need to feel rested and refreshed.
Around 10-years of age, children should receive about 10 hours of sleep per day (Jenni & Carskadon, 2007) while teens are advised to get 8 to 10 hours (Short, Weber, Reynolds, Coussens, & Carskadon, 2018). Yet, on average teens are not sleeping enough (Lui et al., 2017). Because of changes in their circadian rhythm, alterations in hormones, along with academic and social obligations such as sports or employment (Lui et al., 2017) adolescents are more likely to have irregular sleep patterns and experience daytime sleepiness. With advances in technology, kids are especially vulnerable to staying up late at night, often in front of a tv screen or a smartphone. What’s more, exposure to light that is emitted by many screens interferes with our internal clock making it more difficult to fall asleep.
Other changes in aging and sleep are due to how our bodies adapt with our circadian rhythm, and the impact on the metabolism (Laposky, Bass, & Kohsaka, 2008; Wolk & Somers, 2007) as well as the menstrual cycle (Kurth et al, 2010; Lui et al., 2017). For young women, the menstrual cycle greatly influences sleep need. Typically, girls experience a decrease in the time from wake to sleep and a higher need for sleep compared to boys. Excessive daytime sleepiness and the menstrual cycle are associated with increased risk of insomnia (Lui et al., 2017; Noon et al., 2014). Similar to young children, adolescents who get enough “good sleep” perform better in school and experience fewer health problems. It’s important for us to recognize the role of sleep during this hectic moment in life. The reason why a student underperforms or has difficulties in school may potentially have more to do with their sleep rather than their talent and ability.
Sleep for Adulthood
As we age, it becomes increasingly clear that aging and sleep are experienced differently by men and women. Both human and animal studies show gender and hormones (estrogen, progesterone, testosterone) inﬂuence circadian rhythms and are further influenced by environmental, societal and cultural demands (McCoy & Strecker, 2011). It is probably to no surprise that sleep patterns may vary by person (Lauderdale et al., 2006; Nunes et al., 2008; Unruh et al., 2008). In many respects we have some stable features as our circadian rhythm and sleep patterns, yet within those sleep patterns we have fingerprint-like elements that contribute to our ability to achieve “good sleep”. Most adults with a routine sleep schedule and who sleep well should get around 8 hours of sleep (Carskadon & Dement, 2011). Once asleep, adults typically begin in NREM sleep and transition to REM sleep and throughout the night, there is decreased NREM sleep and an increase in REM sleep (Carskadon & Dement, 2011). Throughout adulthood, during NREM sleep, we experience less slow wave or deep sleep in frontal regions of the brain (Carrier et al., 2011). Frontal brain regions are known for cognitive processes such as reasoning and thinking abstractly. Interestingly, young children show increased slow oscillations and research suggests that the increase of these oscillations in frontal areas mirrors the development of cognitive skill sets (Page, Lustenberger, & Frohlich, 2018). Compared to adulthood, one possible reason why we see rapid change in early development, is due to the development of more sophisticated cognitive skills, occuring in frontal brain regions.
Sleep needs are often a subjective sleep measure. Subjective sleep quality is recognized as an important aspect of a healthy lifestyle, with differences reported between males and females. Females report an increased need for sleep and more complaints of non-refreshing sleep than males (Mehta, Shafi, & Bhat, 2015). And, across the lifespan, females are more likely than males to report dissatisfaction with their sleep (Lui et al., 2017;Mehta, Shafi, & Bhat, 2015). For females, sleep seems to be influenced by hormonal factors, with women typically experiencing more sleep disturbances in connection with the menstrual cycle, pregnancy, and menopause (Pines, 2016). Menstrual cycles are associated with well-known changes in reproductive hormones that may influence sleep. Pregnancy also influences sleep needs, that vary depending on the trimester of the pregnancy and whether there are pre-existing health conditions (Mehta, Shafi, & Bhat, 2015).
Due to increased demands of daily life, other factors influence sleep quality. For example, employment and travel greatly affect our sleep. Prolonged wakefulness can be attenuated by increased bouts of deep sleep. In many instances, employment and daily demands of life make it difficult to obtain quality sleep. Males and females may experience increased anxiety as a result of poor sleep and increased sleep difficulties, but not everyone experiences this (Goldstein-Polkarski et al., 2018). For many reasons, sleep loss, sleep disruptions, and anxiety are known to alter cognitive performance, attention, emotional reactivity, and learning (Lustenberger et al., 2012). However, healthy sleep habits can diminish some of these issues.
Sleep for Old Age
Older adults are often thought to not sleep as well as younger adults. Yet, it is unclear if older adults experience less “good sleep” or simply do not require as much sleep (Mandler, Winer & Walker, 2017). At this stage of life, changes in sleep include earlier bedtimes, longer time to fall asleep, shorter sleep duration, becoming easily awakened, and (consequently) more night awakenings (Mandler, Winer & Walker, 2017). Because of difficulties experienced during aging and sleep, older adults experience decreased deep sleep known as slow wave sleep (SWS), fewer NREM and REM transitions, and increased time spent awake throughout the night (Conte et al., 2014; Feinberg and Carlson, 1968; Kales et al., 1967; Klerman and Dijk, 2008; Landolt et al., 1996; Ohayon et al., 2004; Redline et al., 2004; Van Cauter et al., 2000; Vienne et al., 2016). Difficulties in sleep have also been associated with decreases in memory performance and more specifically memory consolidation (Fogel & Smith, 2011).
While not all older adults report difficulties with their sleep, some are more likely to take daytime naps (Foley et al., 2007). Though in early childhood these naps are planned, in older age, these naps are typically unplanned (Foley et al., 2007). One reason for this change is due to alterations in nighttime sleep such as increased night waking and decreased deep sleep. These changes may be why 1 in 4 older adults report daytime sleepiness and thus, the need for a daytime nap. Though difficulties sleeping are widely reported among older adults (Ancoli-Israel et al, 2003; Lauderdale et al., 2014) this is not consistent for everyone. One of the biggest challenges with aging and sleep is understanding what health issues are caused by age, by sleep, or by a combination of the two.
Aging and sleep are a necessary part of life. Understanding how these processes occur and the impact on sleep quality will not only help you become more aware of your needs but may help you understand other important processes and identify ways to achieve a better night’s sleep.