SANTA CLARA, California, February 12. THE last installment of this series tackled a brief introduction to sleep and the different phases. If you missed, Science of Performance: Are Morning Workouts Worth the Sleep Deprivation? Part I , make sure to read about the different phases of sleep. This installment will discuss what initiates the sleeping process and how sleep influences various bodily functions.
What Initiates Sleep
Sleep is a complex and highly integrated function and the initiation of sleep is no exception in this complexity. At the start of sleep, many hormonal changes occur, specifically cytokines and insulin and instead of discussing all these hormones, which would make this series many more than the four intended parts if I discussed all of them.
I will, instead, focus on the likely most important hormone for the initiation of sleep, melatonin. Melatonin is produced by the pineal gland, a small gland located in the center of the brain. Light (and artificial light, computer screens, phones, etc.) inhibits the production of melatonin and is permitted by darkness. This is why, nearly all melatonin secretion occurs at night (Atkinson 2003). At night, melatonin peaks and likely regulates the various sleep phases, particularly increasing the rapid eye movement (REM) phase.
Durations of Sleep Stages
Achieving all the phases of sleep is crucial, as noted last week, these various phases each have a different, yet vital role. Overall, non-rapid eye movement sleep (NREM) accounts for approximately 80% of total sleep and REM sleep makes up the remainder (Carskadon 1994).
Stage 1 of NREM sleep comprises 2 – 5% of sleep, but disruptions of this phase occur with low stimulation. This low stimulation makes walking more frequent during this phase, but luckily, returning to stage 1 sleep is easy and fluctuating sleep occur during this phase (like bobbing between wakefulness and sleep during class).
Stage 2 NREM sleep takes 10 – 25 minutes after stage 1 and is much less sensitive to arousal. Stage 3 and 4 of NREM accounts for 13 – 23% of sleep time and are the deepest sleep stages.
REM sleep results in central motor activation and increases in length as sleep progresses, initiating at 5 – 15 minutes during the first cycle and potentially accumulating to 25 minutes. The highest levels of dream vividness, brain activity, and eye movement occur during this stage of sleep.
The combination of these phases combines to create a sleep cycle. Each cycle of sleep last around 90 to 110 minutes, and typically occur four to six times per night (Gyllenhaal et al., 2000).
Different types of disrupting sleep exist, yet these can broadly be divided into three categories:
* Sleep fragmentation: This is characterized by disruption of the normal progression of sleep stages, as occurs in certain sleep disorders.
* Prevention of sleep stages: As an example of this, certain psychiatric medications can selectively impair REM sleep (specifically benzodiazepines).
* Sleep deprivation: This is the typical result of inadequate sleep volumes and the typical form of disturbed sleep, as many people are more ambitious than they are physiologically capable!
Issues with Studying Sleep
Like all human studies, there are issues with research. For one, self-reported sleep durations are frequently inadequate, as many overestimate their sleep (Ancoli-Israel 2003). Secondly, the volume and quality of sleep are important to distinguish, as many report sleeping 8 hours, but lying in bed may account for a few of these hours. As you will see in later posts, sleep quality is more important than sleep quantity.
These human errors cause many researchers to collect blood samples, but the blood values will be greatly augmented depending on the time of sampling, as many hormones undergo diurnal variation (read Science of Performance: Diurnal Variation in Swimmers). Alterations in dietary intake and the presence of illness and allergies may also alter these blood values.
Disturbed Sleep Prevalence
The International Classification of Sleep Disorders has outlined more than 80 distinct sleep disorders, and 30 to 45% of the U.S. population exhibit sleep and wakefulness problems (Hossain & Shapiro, 2002). Needless to say, sleep disorder prevalence is dependent upon the country investigated and varies widely within sub-populations, such as swimmers.
Using the U.S. in 1998 as an example, 18% of individuals reported difficulties with wakefulness at least a few days a week (Johnson, 1998). Indeed, 9% reported being diagnosed by a physician as having a sleep disorder (Gallup, 1997). To compensate for this problem, 10 to 15% of US respondents reported using over-the-counter stimulant medications or supplements. No sleep studies exist on swimmers, but it is believed the amount of sleep deprivation prevalence is high in populations high in large volumes of wakefulness during darkness (like shift workers and possibly early morning swimmers).
Problems with Sleep Deprivation
A myriad of problems occur during periods of sleep deprivation. The next installment will address the physiological consequences of sleep deprivation. For now, understand the importance of obtaining all the stages of sleep, since all of these phases play a vital role of well-being. Lastly, the ending to this saga will provide practical insights and methods for implementation, as it depends is not an applicable solution!
Dr. G. John Mullen is the owner of of the Center of Optimal Restoration and creator of Swimming Science. He received his doctorate in Physical Therapy at the University of Southern California. G. John has been featured in Swimming World Magazine, Swimmer Magazine, and the International Society of Swim Coaches Journal.