Most adults need at least 7 hours of sleep per night, and the bulk of that time is what sleep trackers and researchers refer to as “core sleep.” This is the uninterrupted, consolidated sleep your body depends on to cycle through its essential repair and recovery stages. If you’re consistently getting less than 7 hours, your body isn’t completing enough of those cycles to keep you healthy.
How Much You Need by Age
The CDC breaks sleep recommendations into specific age brackets. Teens between 13 and 17 need 8 to 10 hours. Adults aged 18 to 60 need 7 or more hours. Adults 61 to 64 should aim for 7 to 9 hours, and those 65 and older do well with 7 to 8 hours.
These numbers represent total sleep time, not just time spent in bed. That distinction matters. If you’re in bed for 8 hours but spending 90 minutes tossing, scrolling your phone, or waking up repeatedly, your actual core sleep might be closer to 6.5 hours. A healthy “sleep efficiency,” the ratio of time asleep to time in bed, falls between 85% and 90%. So if you need 7 hours of sleep, plan to be in bed for roughly 8 hours to account for the time it takes to fall asleep and any brief awakenings during the night.
What Happens During Those Hours
Sleep isn’t one uniform state. Your brain cycles through distinct stages roughly every 80 to 100 minutes, and a full night typically includes four to six of these cycles. Each cycle contains lighter sleep stages, deep sleep, and REM sleep, and each serves a different purpose.
Deep sleep (sometimes called slow-wave sleep) is the most physically restorative stage. Your muscles repair, your immune system strengthens, and growth hormones are released. Adults should spend about 20% of their total sleep in this stage, which works out to roughly 60 to 100 minutes across an 8-hour night. Deep sleep is concentrated in the first half of the night, so cutting your sleep short by staying up late but waking at the same time may not rob you of as much deep sleep. But consistently sleeping only 5 or 6 hours still reduces the total amount you get.
REM sleep, the stage most associated with vivid dreaming, accounts for about 25% of your total sleep time. This is when your brain consolidates memories, processes emotions, and performs a kind of mental housekeeping. REM periods get longer as the night goes on, so the final cycle or two of a full night’s sleep contain the most REM. People who cut their sleep short in the morning tend to sacrifice disproportionate amounts of this stage.
The remaining 55% of your sleep is spent in lighter stages that serve as transitions between deep and REM sleep. These stages aren’t wasted time. They help regulate body temperature, heart rate, and brain activity as you move through each cycle.
What Happens When You Don’t Get Enough
Adults who regularly sleep fewer than 7 hours are more likely to develop high blood pressure, type 2 diabetes, obesity, and depression. The cardiovascular risks are particularly well documented. During normal sleep, your blood pressure drops. When sleep is consistently too short or too fragmented, your blood pressure stays elevated for longer stretches, which strains the heart and blood vessels over time.
Poor sleep also disrupts hunger hormones in ways that make you feel hungrier than usual and less satisfied after eating. This is one reason chronic sleep loss is so strongly linked to weight gain, especially in children and teenagers. Beyond metabolic effects, people who sleep poorly tend to develop compounding unhealthy habits: higher stress, less physical activity, and worse food choices. The damage isn’t just biological. It’s behavioral.
Signs You’re Not Getting Enough Core Sleep
Sometimes the signs are obvious, like struggling to stay awake during meetings or needing caffeine to function past noon. But inadequate sleep doesn’t always look like dramatic sleepiness. Some common markers are subtler:
- Persistent fatigue without obvious sleepiness. You may not be nodding off at your desk, but you feel a low-grade exhaustion that has lingered for weeks or months.
- Increased appetite or cravings. If you’re consistently hungrier than seems justified, disrupted sleep hormones could be a factor.
- Difficulty falling asleep. Paradoxically, not sleeping well can make it harder to fall asleep. Spending too much time on screens before bed is a common contributor.
- Frequent nighttime awakenings. Waking up multiple times, especially after alcohol, fragments your sleep cycles even if your total time in bed looks adequate.
- Waking up gasping or choking. This can be a sign of sleep apnea, a condition where the airway becomes blocked during sleep, which significantly reduces sleep quality regardless of duration.
If several of these sound familiar and have persisted for more than a few weeks, the issue may go beyond simple sleep habits.
How to Improve Your Core Sleep
The single most effective change most people can make is keeping a consistent sleep and wake schedule, including on weekends. Your body’s internal clock relies on regularity, and shifting your bedtime by even an hour or two on weekends can create a kind of social jet lag that takes days to recover from.
Your sleep environment matters more than most people realize. Keep your bedroom cool, dark, and quiet. Light exposure in the evening suppresses the signals your brain uses to initiate sleep, so minimizing screen time in the hour before bed makes a measurable difference. Room-darkening shades, earplugs, or a fan for white noise can all help, depending on your specific environment.
What you do during the day also sets up the night. Physical activity improves both sleep duration and the amount of deep sleep you get, though exercising too close to bedtime can have the opposite effect for some people. Caffeine has a half-life of about 5 to 6 hours, meaning half the caffeine from your 3 p.m. coffee is still circulating at 9 p.m. Alcohol, while it may help you fall asleep faster, fragments sleep in the second half of the night and reduces REM sleep.
If you’re tracking sleep with a wearable device, pay attention to trends over weeks rather than obsessing over a single night’s data. One poor night doesn’t cause lasting harm. A pattern of consistently falling below 7 hours, or seeing very low deep sleep and REM percentages, is worth taking seriously and adjusting your habits around.