Question: Why does a reduction in sleep duration from 9 hours to 7.5 hours result in increased alertness for some individuals?
Waking up feeling refreshed and alert is a goal many of us strive for each morning. However, the quality and timing of our sleep cycles play a crucial role in determining how we feel upon waking.
According to sleep expert Heather Darwall-Smith, the duration of sleep is not the only factor that affects our sense of alertness in the morning. Sleep inertia, a condition that occurs when waking up during deep sleep, can cause feelings of grogginess, heavy-headedness, and slowness.
During a typical night, people cycle through various stages: light sleep, deep (slow-wave) sleep, and REM (dreaming) sleep, usually 4–6 cycles per night. Each sleep cycle lasts for about 90 minutes, and can extend to 100 minutes.
The timing of our awakening in relation to these cycles strongly impacts how refreshed we feel. Waking at the end of a cycle (lighter sleep) tends to make waking easier, while waking in the middle of deep sleep triggers sleep inertia.
For instance, waking after 7.5 hours of sleep might be during a lighter sleep, making getting up easier and feeling more alert. In contrast, waking after 9 hours may happen during deep sleep, causing sleep inertia and making one feel groggy and less cognitively functional.
This phenomenon explains why longer sleep duration does not always mean feeling more rested immediately upon waking—quality and timing of sleep cycles are crucial factors.
To help combat sleep inertia, it's recommended to wake up at the same time every day. This keeps your circadian rhythm steady, making it more likely you'll wake from lighter sleep. Waking in sync with the rise of core body temperature, which is part of the circadian rhythm, feels more natural and can contribute to an easier wake-up.
Using a sleep diary or wearable for 1-2 weeks can help you see when you naturally wake feeling good and find patterns in bedtime, wake time, and total hours. Sleep tracking apps can also help you get a better sense of how you're sleeping through the night, giving insights into how long you spend in each sleep stage and helping you to determine a sleep routine that gives you the amount of rest you need to feel your best in the morning.
Morning light signals your brain to stop melatonin production and raise alertness. To help you feel at your best and brightest in the morning, it's recommended to open curtains, go outside, or use a light therapy lamp within 30 minutes of waking. A recent study showed that getting morning sunlight can lead to better sleep efficiency and fewer nighttime awakenings.
In conclusion, understanding sleep inertia and the importance of sleep cycles can help us make informed decisions about our sleep habits. By waking up at the right time in our sleep cycle and incorporating morning light exposure, we can set ourselves up for a more refreshing and alert morning.
Science reveals that the quality and timing of sleep cycles significantly impact our health-and-wellness, specifically our alertness upon waking. Waking during lighter sleep stages can lead to a smoother awakening and improved cognitive function, while waking during deep sleep triggers sleep inertia, causing feelings of grogginess.
