The rest you get during sleep actually requires a lot of hard work. Once you nod off, your body automatically orchestrates a complicated set of processes designed to restore and repair its systems. This intricate process takes place through four distinct stages of sleep, each of which is associated with different types of brain waves and thresholds of arousal, each of which performs specific functions, from consolidating memories to boosting immune function.
The great news is that all of this unfolds without your awareness or your involvement. Like a complex software program, your body recognizes when you hit “go” (or “sleep”) and then cycles through each stage, seamlessly transitioning from one to the next and back again.
That said, there is plenty you can do to inadvertently interfere with this natural process. Just as running out of battery power will disrupt a software download, waking in the night, because of environmental factors or sleep disorders like sleep apnea, can throw the whole operation into disarray.
It’s important to know, too, that the process can’t be rushed. It takes seven to eight hours, no matter how much you’d like to consolidate that time. Not giving yourself enough hours to sleep can prevent you from getting enough of all the different stages of sleep that your body needs to operate well. It usually ends up costing you more time and attention in the long run.
To help you understand exactly what’s at stake, we offer a tour of your amazing sleep architecture.
The Four Stages of Sleep
Stage 1: Transition into Light Sleep
This is the lightest stage of sleep that occurs as you’re transitioning to slumber. You’re not aware of your surroundings, but you can be easily awakened. You may experience hypnic jerks, those body twitches that often occur alongside a sense of falling. Your body temperature begins to decrease and your muscles relax. For most people, this stage lasts from one to five minutes total. But people with chronic pain or other health conditions who are awakened throughout the night may experience more cumulative stage 1 sleep.
Stage 2: Light Sleep
Lasting 10 to 25 minutes, this is the first stage of true sleep. The workhorse, stage 2 makes up about 45% of your total sleep. Your body is relaxed and your heart and breathing rates slow further. This allows you to maintain sleep while your brain starts processing and sorting through all the information from your day. This is when most of your memory consolidation occurs. Your brain’s electrical activity is irregular during this stage and includes sleep spindles, brief patterns of fast brain waves, and K complexes.
Stage 3: Deep Sleep
Deep sleep is the stage most difficult to awaken from, and it is characterized by delta waves, or large, slow brain waves. Your blood pressure continues to drop, and your pulse slows to 20% to 30% of its waking rate. In this stage, your body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system. Because it’s so important, your body prioritizes deep sleep early in the night, so it’s concluded about four or five hours after falling asleep. When it occurs, sleepwalking happens during deep sleep.
Stage 4: REM Sleep
Rapid eye movement sleep is when dreaming happens. REM cycles get longer later in the night, after your brain has sorted through all the information and left what’s important for you to process and integrate. REM is critical for learning and emotional processing. During this light sleep period, your brain is extremely active, but your muscles are extremely relaxed — essentially paralyzed — so you don’t act out your dreams.
How You Cycle Through Sleep Stages
You move through stages 1, 2, and 3, then REM, and then cycle back to stage 2 and repeat. Most of the deep sleep is completed by hour four, so the later cycles may not contain deep sleep. REM sleep is shorter in the beginning of the night and longer closer to the morning, which is why we can often remember our dreams when we wake up.
Threats to Your Sleep Architecture
Though you can’t control your sleep stages, you can protect them. Not getting enough sleep can reduce the number of sleep cycles, resulting in less deep, restorative sleep.
The most important thing you can do to keep everything running smoothly is to get between seven and eight hours of sleep. Each cycle takes about 90 to 110 minutes, so a good night’s sleep allows for about four cycles.
Sleep stages can also suffer if your sleep is fragmented. A number of conditions can cause awakenings, which sets you back to stage 1. For example, obstructive sleep apnea (OSA) causes you to stop breathing, triggering mini-awakenings throughout the night. This is more common in deep sleep and REM sleep, so OSA may cause you to spend less time in these stages.
Other conditions, like restless legs syndrome, a weak bladder, hot flashes, and depression, can disrupt your sleep stages. Alcohol and certain medications taken for insomnia, like benzodiazepines, reduce deep sleep and REM sleep.
If you think you may have a sleep issue, speak to your health care provider.
