Even after sleeping for 8 hours, I still feel tired. The reason might be the "quality of my sleep."

"I went to bed early yesterday, but I've been feeling sluggish since this morning."
If days like that keep up, it will start to affect your work performance.
Sleep cannot be measured solely by the amount of time spent sleeping. If you don't get enough deep sleep, you may not feel fully rested even after sleeping for 8 hours.
Garmin smartwatches track the balance of deep sleep, light sleep, and REM sleep based on your body movements and heart rate changes during sleep, and display this as a “Sleep Score” out of 100 in the morning. Since you can see numerically whether you slept well or poorly the night before, it becomes easier to recognize how your sleep is related to lifestyle habits such as staying up late, drinking alcohol, and consuming caffeine.
Start by “visualizing” your sleep.
You can try out the sleep feature on an actual device in our store.
We look forward to seeing you at the GARMIN Store in Kobe Sannomiya (Mint Kobe, 6th Floor).