The American Academy of Sleep Medicine and the National Sleep Foundation recommend that adults get 7–9 hours of sleep every night. The Army Field Manual, (FM) 7-22, Holistic Health and Fitness (H2F), echoes these recommendations. But have you ever wondered why 7–9 hours is recommended and where these numbers come from? Learn about the rationale behind recommended sleep duration and its practical implications.
Why 7–9 Hours of Sleep
Sleep experts regularly review emerging evidence on how sleep supports health, wellness, and performance. They assess how different sleep durations correlate with changes in physical, cognitive, and emotional functioning. Their conclusions consistently show that people who sleep substantially less or more than the recommended 7–9 hours tend to have worse health and performance outcomes than people who sleep 7–9 hours.
New evidence on the importance of sleep continues to emerge. Yet the core recommendation that adults consistently get 7–9 hours of sleep each night remains unchanged.
Factors such as genetics, age, and metabolic needs influence your sweet spot for how much sleep you need within this 7–9-hour range. For example, you might have noticed that as you age, you require less sleep. In rare cases, people can do well with a little less than 7 hours or slightly more than 9. But most adults—worldwide—fall within this range.
The 4 Stages of Sleep
Sleep is not a single, uniform entity. It’s made up of 4 main stages that cycle in an organized and structured pattern known as “sleep architecture.”
- Stage 1—Lightest sleep (about 5% of total sleep time): A light transition phase between wakefulness and deeper sleep. In this brief period, muscles relax, and brain waves begin to shift into slower patterns.
- Stage 2—Light sleep (about 45–55% of total sleep time): A deeper stage where body temperature drops, muscles relax further, and brain activity continues to slow in preparation for deeper sleep.
- Stage 3—Deep or slow-wave sleep (about 20–-25% of total sleep time): A stage characterized by maximum muscle relaxation and lowest heart and respiratory rate. Deep sleep supports physical restoration, including tissue repair, immune health, memory consolidation, and many other aspects of health and performance.
- REM Sleep—Rapid Eye Movement (about 20% to 25% of total sleep time): The brain becomes highly active, resembling wakefulness. REM sleep plays a crucial role in supporting emotional regulation, creativity, and memory processing.
A full night’s sleep consists of 4–6 sleep cycles, each lasting about 90 minutes. All stages are important, and you need to sleep long enough to go through all of them. To get the full benefits of each stage, your brain needs multiple uninterrupted sleep cycles, which is less likely to happen if you sleep fewer than 7 hours.
What Happens When Sleep is Cut Short
One of the major problems with sleep loss is it interferes with your sleep architecture. Imagine a cake with 3 layers of cake, 2 layers of filling, and the icing on top. When you eat a whole slice, you generally get a proportional mix of all the layers. If this cake represents your sleep and you eat the full slice (get 7–9 hours of sleep), the layers are distributed proportionally. But cutting your sleep short is like getting a thinner slice and leaving some cake filling (REM sleep) behind.
Some of the effects of sleeping fewer than 6 hours include the following:
- You go through fewer complete sleep cycles, disrupting the ideal ratio among the 4 stages.
- Your brain tends to protect stage 3 sleep at the cost of REM sleep. Your brain tracks when you get less REM sleep than usual. So when you have the chance to sleep longer, your body might try to compensate for lost REM and spend more time in this stage.
- If you don’t get enough sleep on a regular basis, your body doesn't have enough time to recover. A chronic lack of REM sleep can reduce your cognitive performance, such as attention, learning, and working memory.
- Over time, getting fewer than 6 hours of sleep can disrupt your sleep cycles, making it harder to get consistent, high-quality sleep.
Practical Takeaways
How does all this talk about “sleep architecture” and your need to get 7–9 hours apply to you? Here are some key takeaways:
- You have an ideal sleep duration. While 7–9 hours is the general guideline, everyone has a personal sweet spot within that range. Conducting a sleep self-study can help you learn your ideal sleep duration. And that sweet spot isn’t static. You might need some extra sleep if you didn't sleep well the previous night, worked out harder than usual, or are recovering from sickness.
- You don't get to choose how much sleep you need. Most adults need 7–9 hours of sleep. Your genetic makeup determines your sweet spot in this range. Trying to “train” yourself to function on less sleep doesn’t work and frequently backfires in the long run.
- You get used to being tired when you consistently get less than 6 hours of sleep. If you regularly get 6 hours or less of sleep and feel “just fine,” you've likely gotten used to being tired—not adapted to functioning with less sleep. Cognitive and physical performance suffer even when sleep loss feels manageable.
- You can't train for future sleep deprivation by reducing how much sleep you get each night. A common misconception among Service Members is that intentional sleep deprivation during training can help you tolerate sleep deprivation in the field. In reality, cutting sleep beforehand creates a sleep debt, weakens your resilience, and reduces your tolerance to sleep loss during missions.
- Healthy sleep habits can help you improve sleep duration and quality. Check out HPRC’s Sleep Habits Self-check to identify habits that can help you optimize your sleep readiness. If you need help creating a strategy to build healthy sleep habits, read HPRC’s Strategic Habits for Better Sleep.
- Adopt a long-term perspective. Occasional short nights will happen, especially in the military. But as with other aspects of your health, wellness, and performance, perfection isn’t the goal. Focus on controlling what you can, when you can, and developing consistent habits in the long run.
Military operational tempo and environment can add challenges to maintaining regular sleep. Many times, Service Members can't meet their daily sleep recommendations for reasons beyond their control. But when you do have control over your schedule and environment, take advantage of it. Prioritizing healthy sleep habits to get the recommended 7–9 hours of sleep can support your overall readiness and performance.
