Although sleep is a natural part of everyday life, it is integral to health. Not many are aware of how a lack of sleep can cause severe consequences and, in some cases, death. On average, a human can last around eight to 11 days without sleep but suffer from severe declining cognitive functioning. Many experience hallucinations and the inability to concentrate, much less perform daily tasks. Hormones, including the pineal gland and melatonin, are particularly important where sleep is concerned. 

Hormones Hormones

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Table of contents

    Melatonin is the sleep hormone. Let's explore melatonin and the pineal gland.

    • First, we are going to state the hormones definition.
    • We will then provide hormone examples to illustrate the vast amounts of hormones performing functions within your body.
    • Then, we will delve into melatonin hormone function, aiming to understand how melatonin is a hormone from the pineal gland.
    • Finally, we will cover jet lag and Seasonal Affective Disorder (SAD) to explain how bodily rhythms and melatonin interact and how sleep can quickly be affected by outside influences.

    Hormones, man sleeping in his bed dreaming of stars and planets, StudySmarterMelatonin is known as the 'sleep hormone', freepik.com/pch.vector

    Hormones Definition

    Similar to neurotransmitters, hormones are chemical messengers within the body.

    Hormones are released from an endocrine gland, organ, or tissue to be received by target organs and tissues to perform or regulate different functions. The bloodstream carries hormones around the body.

    Hormones examples

    As hormones are chemical messengers, many types of hormones exist within the body that target organs, cells, and tissues.

    Hormones examples include:

    • Adrenaline
    • Melatonin
    • Thyroxine
    • Insulin
    • Glucagon
    • Testosterone
    • Oestrogen
    • Progesterone
    • Oxytocin

    Sleep acts as a reset for hormones, and naturally, certain hormones rise and decline in levels depending on when you are asleep and when you are awake.

    Pineal Gland Hormones

    The pineal gland is located behind the third cerebral ventricle between the two hemispheres in the brain. It is shaped somewhat like a pinecone (hence the name).

    It is an endocrine gland. Endocrine glands are part of the endocrine system, which releases hormones to perform different functions. The hormones travel around the body via the circulatory system and bind to their preferred receptors when they meet the target cell.

    The pineal gland produces melatonin. To understand the role melatonin plays in our sleep, we need to know about bodily rhythms, specifically the circadian rhythm, on which the sleep-wake cycle operates.

    The Sleep-Wake Cycle Summary

    The body operates on internal rhythms (biological clocks), and the sleep-wake cycle is part of the circadian rhythm, a daily rhythm. It is a 24-hour cycle which affects the body's temperature, sleep, feelings of wakefulness, and even hunger.

    The suprachiasmatic nucleus (SCN) is an example of an internal pacemaker (endogenous) within the body that aids the regulation of the circadian rhythm. Many consider it the circadian clock.

    Light enters the eye, and cells in the retina detect light levels. The SCN then monitors these light levels to discern approximate times and regulate the circadian rhythm. High light levels usually mean daytime, whereas low levels usually mean nighttime.

    When we are approaching the end of the day, and it's time to sleep, the body temperature drops, light levels are low, and our internal clock readies us for sleep. The sleep cycle (the four stages of sleep and REM sleep) is an example of an ultradian rhythm (a rhythm that lasts less than 24 hours).

    Hormones, a woman adjusting a clock representing day and night circadian rhythm, StudySmarterThe sleep-wake cycle operates on a 24-hour cycle, freepik.com/redgreystock

    Melatonin Hormone Function

    Melatonin is the sleep hormone. It is produced when it is dark to signal the time for sleep. As discussed above, the circadian rhythm is what the sleep-wake cycle operates on, which is intrinsically linked to melatonin. It acts as a trigger in the circadian rhythm, synchronising it to the time and waking hours as a natural clock.

    Melatonin levels increase in the evening to trigger sleep and reduce towards the end of sleep to allow you to wake up.

    As darkness initiates melatonin release, going to bed and lying in a dark room is a prime example of a sleep-triggering situation. The lack of light will induce the release of melatonin, and you will naturally begin to fall asleep in bed.

    Melatonin operates on light and darkness, so we may have problems with our sleep-wake cycle when we artificially affect these scenarios.

    Using your phone while lying in bed at night can disrupt your ability to fall asleep. The light from your phone will affect the levels of darkness and naturally inhibit the release of melatonin, reducing your chances of falling asleep for a while.

    Jet lag and seasonal affective disorder (SAD)

    Jet lag happens when a person travels from one place to another that has different time zones, usually via plane. Planes are the fastest form of travel over long distances.

    When travelling to a place several hours behind the local time of the place you're leaving, you will tend to feel tired earlier than the time that locals to the destination would sleep.

    For example, the UK is nine hours behind Australia. If you were to travel from Australia and land in the UK at 7 pm Australia time, you would arrive in the UK at 10 am. Naturally, three hours later, at around 10 pm Australia time, you may have started to feel tired, but as you are in the UK, it is only 1 pm.

    Your body is still acclimated to Australia's time, and you will start to feel sleepy in the UK, even though it is the middle of the day.

    Feelings of depression characterise seasonal affective disorder (SAD). It seems to affect most in the winter, and people with SAD often have issues with a lack of energy, difficulty concentrating, and general feelings of sadness, guilt, and hopelessness.

    Although it's relatively apparent, lower levels of sunlight occur in winter.

    The circadian phase-shift hypothesis suggests that the stages or phases of the sleep-wake cycle shift in the day, which causes a circadian misalignment. When this misalignment is corrected by providing doses of melatonin, symptoms of SAD improve, as it causes phase delays or phase advances depending on the misalignment (Lewy et al., 2006).

    It is also suggested that people with SAD have issues with overproduction of melatonin, which induces feelings of tiredness, sleepiness, and a lack of energy those with SAD feel.

    Both jet lag and SAD can be treated somewhat with melatonin medication.

    Hormones, woman and man with a map of the world showing different time zones, StudySmarterJet lag affects your sleep-wake cycle, freepik.com/vectorjuice

    Hormones - Key takeaways

    • Hormones are chemical messengers within the body. Hormones are released from an endocrine gland, organ, or tissue to be received by target organs and tissues to perform or regulate different functions.
    • The pineal gland is an endocrine gland that produces melatonin. Darkness triggers the release of melatonin, detected by the suprachiasmatic nucleus (SCN).
    • Melatonin is the sleep hormone and is essential in regulating and synchronising the circadian rhythm on which the sleep-wake cycle operates.
    • Melatonin levels increase in the evening and during sleep, and melatonin levels lower towards the end of sleep and in daylight.
    • Jet leg and Seasonal Affective Disorder (SAD) show how melatonin and bodily rhythms interact.
    Frequently Asked Questions about Hormones

    What are hormones? 

    Hormones act as chemical messengers within the body. Hormones are released from an endocrine gland, organ, or tissue to be received by target organs and tissues to perform or regulate different functions. Examples include melatonin, testosterone, and oestrogen. 

    What hormone regulates melatonin? 

    Melatonin in itself is a hormone and is triggered by darkness. When it is dark, the pineal gland (an endocrine gland) releases melatonin (the hormone) to induce feelings of tiredness and sleep. 

    Does melatonin affect hormones? 

    Melatonin is a hormone that has an effect on target tissues, organs, and cells. 

    What hormones are produced by the pineal gland? 

    The pineal gland produces melatonin, a key hormone in the sleep-wake cycle, and it is the regulator of the circadian rhythm within animals. 

    What happens when melatonin levels are low? 

    When melatonin levels are low, you will generally feel more awake than sleepy. If they are dangerously or abnormally low, you will have issues with falling asleep (insomnia), feelings of restlessness, and issues with staying asleep. 

    Test your knowledge with multiple choice flashcards

    The pineal gland is shaped like a ______.

    What rhythm does the sleep-wake cycle operate on?

    Melatonin is known as the ____ hormone.

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