1. Stages of Sleep The Sleep Cycle

2. How to Measure Sleep Measuring Sleep -- Scientists measure sleep with the following: Electroencephalogram (EEG) -- a device that measures the electrical activity of the brain. Electrooculagram (EOG) -- a device that measures eye activity. Electromyogram (EMG) -- this device is used to measure the muscle activity that occurs during sleep, and is particularly helpful when attempting to identify REM sleep. Electrocardiogram (ECG or EKG) -- as you most likely know, the electrocardiogram is a measure of heart muscle contractions. Temperature - body temperature is connected with sleep via the circadian rhythms (your body's internal, biological clock).

3. Two Categories of Sleep Non-REM Sleep (NREM) -- There are two main categories of sleep, Non-Rapid Eye Movement or Non-REM (NREM) and Rapid Eye Movement (REM). NREM sleep contains all stages of sleep except REM (there are 5, although this is debatable).

4. Stage 0 Stage 0 - also known as “awake.” In this stage your brain wave activity is composed mainly of alpha and beta wave activity. Typically, it is said that the more beta waves, the more active and "awake" your brain.

5. Stage 1 Stage 1 - this is the transition stage from wake to sleep. It is that stage in which you are aware that you are about to fall asleep, but haven't just yet. Approximately 5% of sleep is stage 1, and is characterized by increased amounts of Theta waves and a reduction in Alpha and Beta waves.

6. Stage 2 Stage 2 - This stage of sleep is the most prominent stage as we spend the majority of sleep time in it. Approximately 45% of sleep time is spent in Stage 2. Stage 2 is characterized by a mix of Theta; sleep spindles, K-Complexes, & some Delta waves. Sleep Spindles -- these are short episodes that look (the EEG that is) like small footballs. K-Complex -- these are the surest indicators of stage 2 sleep. When a K-Complex occurs, there is a sudden rise in wave amplitude (height) so the wave goes high above baseline, and sharp decrease in frequency (so the wave looks very wide).

7. Stage 3 Stage 3 - This is the first stage of what is considered "deep sleep" or Slow Wave Sleep (SWS). We spend approximately 7% of our time asleep in this stage, and it is characterized by approximately 20- 50% delta waves within minimal amplitude. Although it is a type of deep sleep, many consider this a transition stage (much like Stage 1) between stages 2 & 4.

8. Stage 4 Stage 4 - This stage is sometimes considered that "true" slow wave sleep stage. We spend approximately 13% of our sleeping time in Stage 4. The sleeper has definitely entered Stage 4 when there are more than 50% delta waves. Many people believe that we dream only in REM sleep (in fact, some people refer to REM as the dreaming stage. While it is true we have most of our dreams during REM, we also dream in SWS) * Note -- NREM sleep is considered inactive sleep, because during these stages, we have decreased Blood Pressure, decreased Heart Rate (measured with ECG), reduced muscle tension (measured with EMG), and slower eye movements (measured with EOG).

9. Stage 5 REM: Occurs in regular intervals every 60- 90 minutes. REM sleep has its own unique pattern of brain waves. The waves look much like the teeth of a saw (means that the waves are fast and close together) and the pattern looks almost identical to stage 1 or wake. How then can we differentiate between REM and Stage 1 or wake? We rely on the other measures - during REM, muscle activity drops to almost nothing; you are virtually paralyzed during REM. a) dreaming - mostly occurs in REM, approximately 80%, although, as mentioned, we do dream in SWS. b) age - amount of REM decreases with age.

10. How Your Sleep 1) have a rapid decent through the stages, from wake, to Stage 1, Stage 2...all the way to Stage 4 2) then we go back to Stages 3 and then 2 briefly before entering the first REM cycle 3) this first REM period occurs 60-90 min after sleep onset, and is usually very brief (maybe a minute to a few minutes) 4) As the night progresses, we continue to have REM cycles every 60-90 minutes, but length and intensity increases across each cycle. 5) the second half of night is mainly stage 2 and REM.