Stages of Sleep

00:00 When we think about sleep, there are a number of stages and we can understand those by their description what's happening for the patient and changes in the brain. And we evaluate those with EEG or electroencephalography. So let's walk through some of those stages of sleep. The first is the awake stage and we can be awake with our eyes open or awake with our eyes closed. When we're awake with eyes open, the person is alert and active and there are prominent beta waves; low amplitude, very high frequency waves because the brain is thinking in their stimuli and things going on. When we close our eyes, there is a reduction in the stimuli that are entering our brain through the eyes. We see a decreased level of alertness in our relaxation of the brain and accordingly we see the development of alpha waves on the brain as that basal rhythm comes out and there is a reduction in all of that high frequency thinking that occurs when the eyes are awake and open. As we move into sleep, the first part of sleep early in the night is non-REM sleep and the first stage is stage 1.

01:09 This is characterized by a transition from wakefulness to sleep. Patients become somnolent or drowsy during that early phase and this is the lightest sleep.

01:21 Typically the EEG pattern during that stage 1 of sleep are theta waves. As the brain moves from the alpha calm of wakefulness to the deep sleep of delta wave sleep.

01:33 Stage 2 sleep is characterized by patients becoming less responsive and the brain is going deeper and deeper into sleep. The heart rate goes down, temperature begins to decrease, and the body prepares for deep sleep as well as REM sleep. This is the longest stage in terms of the total duration of sleep calculated over the course of the night. And on EEG, we see theta waves. Sleep spindles which are bursts of high frequency, low amplitude activity and K complexes which are large discharges of waves. The sleep spindles help to put the brain to sleep and those K complexes help to maintain arousal in case the body needs to wake up for any reason. And then the brain moves into a slow wave or deep sleep. And this is characterized by stage 3 sleep. In this stage, restfulness occurs. This is the deepest part of sleep. It is much more difficult to arouse individuals from this deep stage 3 sleep. Body tissue growth occurs. This is where the brain and body will repair and this decreases in duration over the course of life with age. Here, we see on the EEG delta waves. These are the highest amplitude, lowest frequency waves as the brain is very calm and relaxed and in deep sleep. Next, we see REM sleep and the majority of REM sleep occurs very late at night after the onset of deeper slow wave sleep. REM sleep is our dream sleep and this is where the body and brain will dream. The EEG shows mixed low voltage pattern. In fact, it looks very much like an awake brain on the EEG. If we look at electroocular recordings, we see rapid eye movements and that's what's characteristic of this phase of sleep. The eyes are moving and we can see that both on the electrodes as well as just looking at a patient's eyes. We see irregular sharply peaked eye movements during this phase.

03:40 And very importantly if we record muscle activity through an EMG. We see atonia of voluntary muscles. We don't need to be acting out our dreams and so during REM sleep the body and muscles have no tone and there is no movement, inactivity due to inhibited alpha motor neurons. In certain disorders like REM behavior disorder, the brain lose and body loses this atonia with the ability to act out dreams, and that's REM behavior disorder. On the EEG, it looks very much like someone who is awake. There are beta waves, those high frequency, low amplitude waves that occur during REM sleep. So let's look at some of those EEG patterns. In normal wakefulness, we see alpha and beta waves. You can see they're very low amplitude and high frequency as the brain is thinking and processing stimuli and many things are happening in the brain. In stage 1 sleep as the brain prepares for sleep, we see theta waves, slightly higher amplitude, slightly lower frequency as the brain moves in to a slower wave of sleep. In stage 2, there is this competition between sleep promoting signs and wakefulness promoting activity. The sleep spindles are these really high frequency low amplitude spindles that put the brain to sleep and the K complexes are arousal complexes all occurring on top of those theta waves. And findings on the EEG that appear as you see here are indicative of stage 2 sleep.

05:18 Slow wave sleep that you can see here is very high amplitude, low frequency waves that give this stage of sleep its name of slow wave sleep and here we're looking at delta waves. And then as we move into REM sleep, we see the brain really looks awake. We see the return of beta waves, low amplitude, high activity as the brain is processing all the things that are happening during those dream states. Now let's talk about some of the environmental factors that are associated with sleep and impairments in sleep. Alcohol can significantly change sleep and reduce the amount of REM sleep that the body will experience over the course of night. Caffeinated drinks change our ability to get to sleep and maintain a high quality of sleep. Large meals before bedtime change the ability of the brain and body to get in to that REM sleep and the duration of slow wave sleep. Exercise can impact sleep. Napping during the day can impact sleep. Light exposure around bedtime also changes activity that suprachiasmatic nucleus and can alter the ability to get to sleep.