WHAT IS HEALTHY SLEEP?
Interest in healthy sleep has boomed! Sleep’s critical role for short-and-long term physical, cognitive, and psychological health is universally recognized. Yet, what exactly is healthy sleep? Generally, when discussing healthy sleep, the focus lies on the amount or duration of sleep that one is achieving. However, healthy sleep is more than just achieving a sufficient sleep duration. Healthy sleep is a multidimensional construct that involves achieving sleep of healthy duration, continuity, quality, and schedule consistency.  Below I provide a foundational orientation to these four pillars of healthy sleep, including characterization and importance of each pillar as well as a general, actionable recommendation in bolded text.
PILLAR #1: SLEEP DURATION
Whenever I ask a stranger what they know about sleep, the first response is generally something to effect of “Well, I know that you need 8 hours of sleep each night.” This is both right and wrong. To start, let’s draw attention to the fact that the amount of sleep a human needs changes drastically across the lifespan.  For example, the National Sleep Foundation makes the following recommendations: 
Sleep duration occurring outside of these recommendations (i.e., too little or too much) associates with negative, short-and-long term health implications. Clearly, the answer is not as simple as merely 8 hours of sleep each night… The age-related differences in sleep duration highlight interindividual differences (“differences between persons”) in sleep need. Other factors, such as genetics, lifestyle habits, and medical complications, will also contribute to interindividual differences in sleep need. But, there are also intraindividual differences (“differences within a person”) in sleep need! Stated otherwise, the amount of sleep you need per night will vary night-to-night. Factors such as illness, physical activity, cognitive effort, psychological stress, and emotional turmoil contribute to intraindividual differences in sleep need. Ultimately, it is not possible to provide a precise number of minutes or hours to ensure optimal restoration from sleep.
An effective strategy is to prioritize a sleep opportunity window (i.e., the time that you allocate to sleep each night) that allows for a sleep duration within the recommended ranges. Yet, it is essential to act as your own “scientist” to determine the optimal duration for you that results in you waking up feeling refreshed, while recognizing that there will be variability in your sleep need across nights.
PILLAR #2: SLEEP CONTINUITY
Sufficient sleep duration is certainly a necessary component of sleep health, but true sleep health achieves sufficient sleep duration paired with healthy sleep continuity.  Sleep continuity can be conceived as one’s ability to initiate and maintain sleep across the period that they are attempting to sleep. Sleep continuity measures include sleep onset latency, wake after sleep onset, sleep efficiency, and number of awakenings > 5 minutes in duration.  The table below provides a description of each measure and the National Sleep Foundation’s recommended thresholds for determining healthy and poor sleep continuity. 
Many clinical sleep disorders, such as insomnia, obstructive sleep apnea, narcolepsy, restless legs syndrome, and periodic limb movements disorder, are defined by poor sleep continuity. Similarly, a variety of medical problems (i.e., chronic pain) can cause sleep continuity problems, while poor sleep continuity also increases risk for developing medical problems (i.e., Alzheimer’s Disease).
Often, people with poor sleep continuity will increase their time in bed to try and achieve sufficient sleep duration. Yet, this is problematic for two reasons. First, spending more time in bed awake often perpetuates the problem and leads to more time awake in bed. Second, this approach does not resolve poor sleep continuity, which inhibits ability to access the deeper stages of sleep. As you’ll see in the next section, not all sleep is created equal and the ability to achieve the deeper stages of sleep is necessary for healthy sleep quality.
You may be thinking to yourself, “Got it. I’ll just work to have sleep without any interruptions.” Well, your head is in the right direction, but it is actually an unreasonable expectation to have fully uninterrupted sleep (i.e., Sleep Efficiency = 100%). In fact, it is completely normal and healthy to have 1-2, brief awakenings during a primary sleep period, and the normal number of awakenings increases as we age.  The ability to fall asleep immediately and sleep uninterrupted entirely across a sleep period may actually be an indication of a problem (i.e., chronic sleep deprivation), rather than healthy sleep!
Given the expectation of some interruption during sleep, achieving a sufficient sleep duration requires recognition that we must prioritize a little bit more time in our sleep opportunity window to account for the normal bouts of wakefulness during our sleep period. For example, I strive for 7.5 hours of total sleep time per night and assume healthy sleep continuity. To regularly achieve this amount of sleep, I budget 8 hours for my sleep opportunity window.
PILLAR #3: SLEEP QUALITY
Sleep is fundamentally organized into two categories: Non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is further separated into three stages (NREM1, NREM2, and NREM3). These are currently recognized as the four distinct sleep stages, based on the American Academy of Sleep Medicine.  Different stages of sleep serve different functions. For example, NREM3 is uniquely vital for achieving restoration from sleep.  As mentioned above, not all sleep is created equal!
The amount and timing of different sleep stages is an important component to sleep health, but a true definition of healthy sleep quality does not exist.5 Generally speaking for young and middle aged adults, a night of healthy sleep is expected to contain roughly ≤ 5% of NREM1, 50-55% of NREM2, 15-25% of NREM3, and 15-25% of REM.5 However, these ranges are highly influenced by your individual characteristics, such as age and sex, the presence of sleep, psychological, and/or medical disorders, and lifestyle habits (i.e., physical activity). Unfortunately, the influence of these factors inhibits the ability for precise recommendations.
Ultimately, healthy sleep quality is an amorphous construct at this current time, despite its importance. However, if you are achieving healthy sleep duration and continuity while experiencing nonrestorative sleep, then it is possible that poor sleep quality is the cause and evaluation by a trained sleep professional should be pursued.
PILLAR #4: SLEEP SCHEDULE CONSISTENCY
A final pillar of sleep health pertains to the consistency or variability of a sleep schedule, which refers to the degree to which your bedtime and risetime are maintained. Before we dive into details, it is critical to note that humans are not robots! Slight variation in bedtime and risetime is expected in healthy sleep. In fact, it is encouraged, especially in terms of bedtime. Approaching bedtime and risetime too rigidly (i.e., “I must go to bed at this time and get up at this time”) is likely to interfere with sleep ability and quality. However, keeping the variability minimal is essential for healthy sleep.
Persons performing shift-work provide a direct lens into the damaging effects of sleep schedule inconsistency.  Persons with shift-work jobs are at heightened risk for a multitude of health complications and the variability of their sleep schedule is primarily to blame. Yet, sleep schedule variability, and its associated negative outcomes, translates across the general population. It is common for persons to have drastic variation (i.e., multiple hours) in bedtime and risetime between school/workdays and free days. This variation is known as social jet lag, which contributes to negative outcomes such as poor academic and occupational performance, metabolic, cardiovascular, and psychological disorders, and overall degraded health.    Thus, a critical component of sleep health is the ability to maintain a relatively consistent sleep-wake schedule. But, what is reasonable variation?
Humans are not robots and sleep schedule variation is inevitable. Yet, we have control over the degree and frequency of variation. Do your best to limit variation in your bedtime and risetime, across all days in a week. This is best accomplished by maintaining a consistent risetime, which can be reasonably controlled with an alarm. Maintaining a consistent risetime will in turn lead to a relatively stable bedtime. However, if risetime variability is inevitable, then do your best to limit to < 1 hour from your traditional risetime.
SUMMING UP THE PILLARS OF HEALTHY SLEEP
Hopefully at this point it is clear that healthy sleep is much more than healthy sleep duration. Healthy sleep is the intersection of four pillars: Sleep duration, continuity, quality, and schedule consistency. As sleep health becomes a priority in your life, identify which pillars are out of balance, make the appropriate changes, and reap the benefits! Afterall, you can’t have good overall health without healthy sleep.