Levels of Sleep

1. Transition to sleep

This is usually the first stage of the sleep cycle which lasts about 5 to 10 minutes. In this stage, eyes are closed, muscle activity slows down and a person is easily awoken. It is in this stage as well where some people may notice a feeling of falling which is caused by the sudden contraction of the muscles. Waking up in this stage of sleep will cause a person to feel that he/she has not slept.

2. Light Sleep

This is known as the first stage of true sleep which lasts from 10-25 minutes. The muscles in the body at this stage fluctuates between toning and relaxing. Heart rate slows down and body temperature decreases.

3. Deep Sleep

Also known as slow-wave or delta sleep, it is at this stage of sleep where the body repairs and recovers itself. Brain waves at this stage are extremely slow as blood flow is directed away from the brain to different parts of the body for tissue, bone and muscle regeneration. Hence, if a person is awoken in this stage of sleep, he/she may feel groggy or disorientated for a few minutes.

4. REM sleep (Rapid Eye Movement) / Dreaming

Sometimes known as paradoxical sleep, REM sleep usually occurs 70-90 minutes into a sleep. It happens in cycles lasting for approximately 10 minutes initially to 1 hour. At this stage, our eyes starts to move rapidly, our breathing becomes shallow as our heart rate and blood pressure increases. Intense dreaming is common as a result of heightened brain activity while the body remains immobile or paralysed. As we grow older, the time spent in REM sleep declines from 50% of our sleep for infants, to 20% of our sleep for adults.

The diagram above indicates brainwave activity during the different stages of sleep. As we can see, the wavelength is greatly increased during deep sleep as compared to the other stages. Therefore, for a person to be woken up during a deep sleep, it will take some time for reorientation to return to a relaxed/waking brainwave pattern.

The Homeostatic Component of Sleep

The Homeostatic Component depicts the need for sleep in relation to being awake and being asleep. It tells us that the longer that we are awake, the higher the pressure it is for us to need sleep. The Homeostatic mechanism regulates the sleep intensity. Meaning that the higher the pressure, the greater the intensity will be of the sleep.

The graph on the right shows that the need to sleep increases when we are awake and decreases rapidly when we are sleeping. If we however increase the amount of time we sleep, our sleep cycle gets shorter with our need to sleep becoming lesser.

We may think that it is a good thing to have more sleep and shorter sleep cycles since our body will be well rested. However, the opposite is true. Deep sleep is directly proportionate to the intensity of our need to fall asleep. As the pressure/need to sleep decreases, the amount of deep sleep we receive dissipates. Therefore, we often feel lethargic and tired when we sleep too much due to our bodies not being able to regenerate itself with the lack of deep sleep.

Sleep Deprivation

The graph above shows three different sleeping patterns. The blue line depicts the normal sleep pattern while the red line depicts a person awake for 40 hours and lastly the green line depicts a person who takes a 2 hour nap during the day.

As we can see, our need to sleep increases as our time awake increases. The intensity of the need to sleep increases with the lack of sleep as seen in the red graph. However, the amount of time required for recovery during the time asleep remains the same or similar in all the three scenarios. The only difference is the intensity in which the recovery takes place. Hence, from the graph, we can see that a sleep deprived person will undergo a longer period of deep sleep in his/her 8 hours to ensure sufficient recovery. As for the person who took a 2 hour nap in the day, the intensity of the need to sleep is lesser and the amount of recovery is reduced with the reduced amount of deep sleep.

The Circadian Cycle

Many of us are familiar with the Circadian Cycle. It represents our level of alertness throughout the day. It takes into account, biological elements such as our body temperatures, heart rate and blood pressure.

These elements affect the our level of alertness during the course of a day. It is affected by our daily activities such as mealtimes, exercise and level of stress. The graphs above shows the normal results of an average person during a normal day. These elements co-relate and when put together, plots our the Circadian Rhythm with which our level of alertness is affected.

As we sleep, our heart rate is lowered and hence as seen on the graph, our level of alertness is reduced. Other things that may reduce our level of alertness also includes that of our blood pressure which is often lowered after meal times.

As pilots flying across different time zones through the night, we are not able to fully apply the Circadian Cycle without understanding or adjusting it. By understanding how our alertness is affected by our blood pressure, heart rate and body temperature, we can then make adjustments to our diet and sleep to ensure a safe level of alertness in the cockpit.

Application

Understanding the different stages of sleep and the importance of deep sleep, we can plan ahead to ensure we receive the required amount of deep sleep for recovery. On the other hand, we can also adjust our sleeping hours and patterns based on the understanding of the Homeostatic component of sleep. For example, if there is a need to operate a flight in the middle of the night, a 2 hour nap in the early evening can be taken to ensure sufficient rest to lower our need to sleep during the flight. Also, we know that within 2 hours, we are able to receive a decent amount of deep sleep.

We can also take precaution, knowing that if we sleep for short periods, we may not be lowering our heart rate enough to rest properly. Also, if we wake from a sleep when we are in the deep sleep stage (approximately 40 minutes into sleep), our level of alertness will be greatly reduced as our brain waves take time to reorientate itself. This will become more crucial when operating multiple crew details with rest periods on board a flight.

Finally, since pilots are not able to fit their schedules into the Circadian cycle or Homeostatic cycles due to the requirements in our roster, we have to understand what affects these cycles and make the necessary changes to adjust these cycles to accomodate our next flight to ensure we get a good rest and remain alert.

References