This is probably why you still feel tired even after eight hours of sleep, according to a sleep doctor

Eight hours is often considered the gold standard for sleep. But if you’ve found yourself waking up feeling groggy even after hitting this target, a sleep doctor might have the answer.

According to Dr Alanna Hare, sleep doctor and Sealy sleep expert, everyone will have different sleep needs, but the hours you spend snoozing are only one part of the puzzle.

“Sleep duration gives your body the opportunity to rest—but sleep quality determines what your brain does with that time and impacts how you feel when you wake up in the morning,” Hare tells Fit & Well.

Good quality sleep will see you move through different sleep stages and cycles during the night.

One sleep cycle lasts around 90 minutes, during which the brain will move through light, deep and REM sleep stages—all are important for memory formation, learning, emotional processing and physical repair.

Most adults will have four to six of these cycles each night. If these cycles are interrupted regularly, it could leave you waking up feeling more tired the next day.

“If your sleep cycles are repeatedly interrupted, your body can’t perform essential overnight processes to leave you feeling rejuvenated in the mornings,” Hare explains.

There are lots of reasons why your sleep cycles might get interrupted, but one that’s often overlooked is not maintaining a consistent routine of when you go to sleep and when you wake up.

It can be tempting to stay up late and watch another episode of your favorite Netflix series, or sleep in on the weekends, but, , according to Hare, waking up at the same time every day and going to bed at the same time every night could be the easiest way to improve your sleep quality.

“Your body clock thrives on regularity, and having a regular sleep and wake routine often improves how rested you feel more than chasing a perfect number of hours each night,” says Hare.

Creating a daily routine that supports sleep shouldn’t be overlooked either.

Limit caffeine after lunch because it can delay sleep onset, keep any naps short and early or you can weaken your natural drive to sleep, and focus on relaxing before bed.

Another small change is aiming for a temperature in your bedroom of 65-68°F, which Hare says meaningfully improves sleep quality.

“Overheating reduces slow wave sleep while being too cold at night increases wake-ups,” says Hare.