Best time to go to bed to get a good night's sleep

Dr Karan Raj has shared a sleep tip if you're always waking up groggy despite a full night's sleep. Picture / Supplied

Do you wake up groggy in the morning even when you've gone to bed early?

It could be because you're waking up in the middle of a sleep cycle, which can leave you feeling lethargic even when you've got the recommended seven or eight hours the night before.

British NHS surgeon Dr Karan Raj, who has more than 3.4 million TikTok followers, shared how mastering your sleep cycle can help you feel more refreshed every morning.

"Every night your brain moves through several sleep cycles," he said in the video. "Each cycle starts with light sleep, then you enter deep sleep, then dream, then back to light."

As each of these cycles lasts around 90 minutes Raj advised timing your alarm for the end of a sleep cycle.

"You feel most refreshed if you wake up at the end of these 90 minute cycles," he said. "It's because you're closest to your normal waking state."

To work out what your bedtime should be, Raj said you first need to chose what time you want to wake up.

"Work backwards in 90 minute blocks until you get to a time closest to when you want to sleep," he said.

Raj's video has been viewed more than 1.6 million times, with many people saying the theory made perfect sense.

"This is nuts. I work night shift for three years, go to bed at 6am, wake up at noon," one person commented.

"Almost six hours of sleep exactly every night."

"Would explain why I still feel tired after eight hours of sleep," another wrote.

The 90-minute sleep rule has been shared before and while there's plenty of anecdotal evidence that it works, experts warn against taking it as fact.

Australian Sleep Health Foundation chair Emeritus Professor Dorothy Bruck wrote in 2018 that the advice was problematic given not everyone's sleep cycle lasted 90 minutes.

It's also hard to predict how long it takes to fall asleep at night and time spent waking up during the night is often unpredictable, she said.

"So the best advice for waking refreshed in the morning is to follow good sleep habits," Prof Bruck said.

"Pamper your body clock with routine sleep/wake times. Don't for example, get too little sleep during the working week and then sleep in for hours on a weekend – you will just wake up to your alarm on Monday morning feeling like you need more sleep."

In another video Raj shared his other "secret sleep trick" – how keeping a window open can also help you get quality shut eye if you wake up feeling "groggy and tired".

"It could be because of poor ventilation in your bedroom. Closed rooms with minimal ventilation the CO2 levels will naturally rise and build up," he explained.

"Eventually the carbon dioxide could rise so it effects your REM cycles, the deep sleep stages we need to restore energy."

Higher carbon dioxide levels will also increase the amount of times you wake up, as well as the quality and depth of your sleep.

Leaving your window open will also help you keep cool and sleep better.

"One of the triggers we need for a deep sleep is a drop in our core body temperature," Raj said.