This is because our brain is once in a very deep sleep and then in very shallow one. The better we find the interval between the cycles, the more refreshed we feel after awakening.
The phases of sleep are closely related to the different states of consciousness in which the brain operates at different brainwave frequencies.
- Before we go to sleep, our brain is working at beta frequencies (12-28 Hz). This is a typical standby for normal daily activities. When we go to sleep, the brain activity decreases. The mind calms down and the brain begins to work at alpha frequencies (8-13 Hz). These waves are responsible for the state of relaxation. They are activated before sleeping and after waking up, sometimes during the day, for example once we ponder on something. This period of falling asleep lasts from about 10 to 15 minutes.
- The beginning of Phase 1, when the brain goes from alpha waves to theta waves (4-7Hz). There are sounds detached from each other, illogical associations in our mind. These are hipnagogies that always accompany us when falling asleep. Gradually we lose touch with the reality. Phase 2 is also accompanied by theta waves – we fall asleep, the consciousness is turned off.
- Phase 3 is a transitional phase in which the brain is slowing down its work and is moveing from theta frequencies to delta frequency (05 - 3 Hz). It is working in this way untill the end of phase 4. Both of these phases are characterized by a deep, slow wave sleep. Breathing becomes regular, blood pressure decreases, muscle tension and body temperature decrease as well. The body is resting. If someone wakes you at this point, you'll be very sleepy and distracted, and you won’t remember this fact in the morning. Initially, deep sleep takes up to 60 minutes, and the closer is the morning, the shorter is this phase.
- The brain waves are starting to accelerate and the brain undergoes all the phases in reverse order, starting from Phase 4 (here, instead of Phase 1, REM appears). After 90 minutes of falling asleep we find ourselves again in a shallow sleep. The heart rate and breathing accelerate. The REM (Rapid-Eye Movement) Phase begins. Its name comes from the rapid movements of eyeballs. Some call this phase a paradoxical sleep because the brain activity is then very high as in this moment we experience dreams. Everything that happens in the dream world seems to be very real so the brain is almost as active as during a day.
- A sleep paralysis appears - our muscles are paralyzed. Otherwise, the movements we make in a dream, would be made also in bed. If awoken at this phase, we will certainly remember a dream that we’ve just dreamed. When this phase is complete, the brain starts the cycle again.
As presented in the diagram below, the cycle always lasts for about 90 minutes. The course of sleep looks like this: 1,2,3,4,3,2, REM, 2,3,4,3,2, REM, 2,3,4,3,2, REM? It should be noted, however, that the longer we sleep, the more the relative proportions of different phases in the cycle change. The closer to the morning, the shorter is the phase 4 and the longer is the REM.
The conclusions are that the easiest way to get up is when we wake up between the cycles (and preferably at the beginning of a new cycle) during the REM phase. Then, the brain activity is very high, almost like during a day. If awoken at this point, we are immediately ready to start a new day.
Knowing that entire cycle lasts for 90 minutes, plan your sleep so that its length will be the multiple of 90 minutes. Set the alarm clock so that you can sleep 6 or 7.5 hours (add to it the time that you need to fall sleep). You'll see that when you wake up at the end of the cycles, you'll be rested and full of energy. It will make getting up much easier. Now you can see that the feeling of being well-rested doesn’t necessarily depend on the length of your sleep, but mainly on the point when you wake up!
Of course, for different people the cycle length may be slightly different. Therefore, when sleeping for 6 or 7.5 hours you can’t find the break between the cycles, test the other options, such as 6.5 hours. I guarantee that when you wake up you'll always know whether you woke up just between cycles, or from a deep sleep. After determining the optimal length of your sleep, you'll always be able to wake up well-rested and full of energy :)
Let me know in the comments section how it works for you!