You wake up at 7 a.m. and you feel refreshed.

You decide to sleep in for 2 more hours, and when you wake up at 9 a.m., you feel like crap.

How come those extra 2 hours, instead of energizing you, they actually made you feel worse?

Well, first we need to understand how our sleep works, so we can figure out how to manipulate it and wake up alert and refreshed.

Our sleep is made of three stages.

The first stage is called "light sleep."

This is right after you go to sleep.

Any sudden noises may awake you, and people often report that they feel awaken when sleeping in this stage.

The second stage is "deep sleep" or "Delta Phase."

This is where we sleep most deeply.

The last stage is called "REM Sleep" or "rapid eye movement sleep."

This is the stage where we dream.

Our eyes dart back and forth beneath closed lids and our muscles temporarily shut down to prevent us from acting out dreams.

After the REM sleep, we enter the first stage, light sleep, and the cycle repeats.

The whole cycle takes around 90 minutes to complete.

So if you were sleeping for 9 hours, that means you've been through 6 sleep cycles.

The reason we feel groggy and tired, is when we wake up in the middle of the cycle.

So if you want to feel refreshed, find out when you want to be awake, then count backwards in 90-minute blocks.

For example, if you need to wake up at 7 a.m., you should aim to be asleep at 11:30 p.m.

That's 7 and a half hours of sleep, or 5 cycles. Of course, you can plan for 6 or 9 hours of sleep, but that depends on you.

And also, the common belief that we should sleep for 8 hours per night is actually wrong, because it does not align with our sleep cycles.

So, if you want to never again feel tired or groggy in the morning, and if you want to wake up alert and refreshed, keep in mind the 90-minute rule.

Anyway, don't forget to subscribe, and thanks for watching.