Learn about your sleep
What is sleep?
Although we spend nearly 1/3rd of our day sleeping, a proper definition for sleep is not available. Kryger, Roth and Dement textbook describes it as “a state of behavioral quiescence, where you are unaware and unresponsive to the environment.”
What are the states of consciousness?
Table 1. Characteristics of the 3 Brain States
| Wakefulness | NREM Sleep | REM Sleep | |
|---|---|---|---|
Eye Movements | Present | Slow | Rapid |
Alertness | Alert and aware of
environment | Not alert or aware of environment | |
Brain activity | Active | Not active | Active |
Dreams | None | None | Present |
Motor Activity | Able to move | Muscles relaxed,
no movement | Muscles are paralyzed
(twitches can be seen) |
BP, HR,
Respiration,
Body Temperature
Regulation | Changes based
on activity | Body is at rest and
all are reduced. | Variability is present.
Body is poikilothermic* |
Summary | You are awake, able to
move and respond to
environment. “Active
mind in an active body.” | You are asleep; brain
and body are quiet and
resting. “Quiet mind in
a quiet body.” | You are asleep; brain is
active and muscles are
paralyzed. “Active mind
in a paralyzed body.” |
*absence of body temperature regulation
Table 1. Sleep Cycle
Sleep stages are organized by cycles across the night. When you lie down to sleep, you will first transition from relaxed wakefulness to NREM stages 1 and 2 (light sleep) to NREM stage 3 (slow wave or deep sleep). You will then transition through stage 2 to REM sleep, completing the first cycle. Each cycle is between 60-120 minutes long, and 4 -6 such cycles will be seen throughout the night.
Slow wave sleep is seen prominently in the first third of the night. Similarly, REM sleep becomes progressively longer during the final third of the night. Therefore, your sleep is deeper in the first third of the night and your dreams are more prominent in the final third of night.
Circadian Rhythms
Every cell and organ in your body has an internal clock which generates an internal rhythm. For instance, there are 24-hour rhythms for breathing, heart rate, gastric acid, hormonal secretion and even cell rejuvenation. Sleep–wake cycle is the most prominent of these rhythms. These ‘peripheral clocks’ are controlled by the SCN (suprachiasmatic nucleus), your master circadian clock.
RHT: Retinohypothalamic Tract
SCN: Suprachiasmatic Nucleus
Balance between the C (circadian) and H (homeostatic) drives determines when you sleep and wake-up in the 24- hour period. C drive depends on the SCN, which in turn is entrained by the light-dark cycle, while the H drive depends on how long you have been awake.
C drive: When light enters the eye, it stimulates melanopsin containing ganglion cells of the retina transmitting electrical signals through RHT to the SCN, increasing alertness. At the same time, melatonin production by the pineal gland (sleep modulating hormone) is supressed. Similarly, when the sun goes down, SCN activity is reduced and melatonin production is suppressed, resulting in reduced alertness. This sends a signal to the brain that it is time to sleep.
Therefore, when the sun rises, your alertness will increase and when the sun sets, you will slowly become sleepy, so that you can fall asleep a few hours later.
H drive increases when your body and mind are active. It is low when you wake up in the morning and increase slowly though the day., as a result of accumulative brain activity. It reaches its maximum level at bedtime.
Table 3. Circadian and Homeostatic influence on sleep and wake.
C: circadian drive, H: homeostatic drive
Both C and H drives are low when you wake up in the morning. It may take several minutes or more before your C drive increases for you to feel alert. Both C and H drives increase throughout the day. Around 1-2pm, you may feel sleepy since the H drive is high and your C drive takes a ‘small dip’. This is known as “siesta time” which lasts about 20 – 30 minutes, after which the C drive rises, allowing you to feel alert again.
The H drive reaches its highest level at night and the moment your C drive falls, you will feel sleepy and ready for bed. Once you fall asleep, your H drive will slowly fall. Between 2-3am, the H drive is low and can no longer keep you asleep, but you are able to stay asleep due to very low C drive.
Therefore, during the first part of the day you are able to wake up due to a low H drive and during the second half of the day, you are able to stay awake due to high C drive. Similarly, in the first half of the night, you are able to fall asleep due to a high H drive, and you are able to stay asleep in the final half of the night due to a low C drive.
Knowledge of Circadian and Homeostatic systems can help you feel more alert and sleep better:
1) If you are feeling tired when you wake up, get out of bed right away and turn on lights or get sunlight exposure so that your C drive goes up faster.
2) You may experience tiredness between 1-2 pm (siesta time). A short power nap will help you feel more refreshed. However, even if you don’t nap, you will start to feel better after 20-30 minutes when the C drive goes up.
3) Excessive napping will reduce your H drive, making it difficult to fall asleep at night.
4) If you are working in shifts, you will have low alertness around 3 am. Try to move around at this time so that you do not fall asleep.
What is sleep?
What are the states of consciousness?
Although we think of our mind as awake and asleep, there are actually three states of consciousness: Wakefulness, NREM (non-rapid eye movement) and REM (rapid eye movement) Sleep. The three types of sleep are very different from each other. Table 1 summarizes the differences.
*absence of body temperature regulation
Table 1. Sleep Architecture
Sleep stages are organized by cycles across the night. When you lie down to sleep, you will first transition from relaxed wakefulness to NREM stages 1 and 2 (light sleep) to NREM stage 3 (slow wave or deep sleep). You will then transition through stage 2 to REM sleep, completing the first cycle. Each cycle is between 60-120 minutes long, and 4 -6 such cycles will be seen throughout the night.
Slow wave sleep is seen prominently in the first third of the night. Similarly, REM sleep becomes progressively longer during the final third of the night. Therefore, your sleep is deeper in the first third of the night and your dreams are more prominent in the final third of night.
Circadian Rhythms
Every cell and organ in your body has an internal clock which generates an internal rhythm. For instance, there are 24-hour rhythms for breathing, heart rate, gastric acid, hormonal secretion and even cell rejuvenation. Sleep–wake cycle is the most prominent of these rhythms. These ‘peripheral clocks’ are controlled by the SCN (suprachiasmatic nucleus), your master circadian clock.
Table 2. Circadian system
Balance between the C (circadian) and H (homeostatic) drives determines when you sleep and wake-up in the 24- hour period. C drive depends on the SCN, which in turn is entrained by the light-dark cycle, while the H drive depends on how long you have been awake.
C drive: When light enters the eye, it stimulates multiple receptors of the retina, creating electrical signals with different functions. Signals generated by the photoreceptors (rods and cones) generate images in the brain, while signals generated by melanopsin containing ganglion cells travel through the RHT to increase SCN activity.
When the SCN is active, it increases alertness, and suppresses production of melatonin by the pineal gland (sleep modulating hormone). Similarly, when the sun goes down, SCN activity is reduced, resulting in reduced alertness. In addition, the pineal gland is no longer suppressed so it produces melatonin, which signals to the brain that it is time to sleep.
Therefore, when the sun rises, your alertness will increase and when the sun sets, you will slowly become sleepy, so that you can fall asleep a few hours later.
H drive increases when your body and mind are active. It is low in the morning when you wake up after a full night of rest, and will increase slowly though the day, as a result of accumulative brain activity. It reaches its maximum level at bedtime.
Table 3. Circadian and Homeostatic influence on sleep and wake.
C: circadian drive, H: homeostatic drive
Both C and H drives are low when you wake up in the morning. It may take several minutes or more before your C drive increases for you to feel alert. Both C and H drives increase throughout the day. Around 1-2pm, you may feel sleepy since the H drive is high and your C drive takes a ‘small dip’. This is known as “siesta time” which lasts about 20 – 30 minutes, after which the C drive rises, allowing you to feel alert again.
The H drive reaches its highest level at night and the moment your C drive falls, you will feel sleepy and ready for bed. Once you fall asleep, your H drive will slowly fall. Between 2-3am, the H drive is low and can no longer keep you asleep, but you are able to stay asleep due to very low C drive.
Therefore during the first part of the day, you are able to wake up due to a low H drive and during the second half of the day, you are able to stay awake due to high C drive (even though H drive is also high) keeping you awake. Similarly, in the first half of the night, you are able to fall asleep due to a high H drive, and you are able to stay asleep in the final half of the night due to a low C drive.
Knowledge of Circadian and Homeostatic systems can help you feel more alert and sleep better:
1) If you are feeling tired when you wake up, get out of bed right away and turn on lights or get sunlight exposure so that your C drive goes up faster.
2) You may experience tiredness between 1-2 pm (siesta time). A short power nap will help you feel more refreshed. However, even if you don’t nap, you will start to feel better after 20-30 minutes when the C drive goes up.
3) Excessive napping will reduce your H drive, making it difficult to fall asleep at night.
4) If you are working in shifts, you will have low alertness around 3 am. Try to move around at this time so that you do not fall asleep.
