Natural Causes of Fatigue and Drowsiness

Drowsy Driving: The Causes of Fatigue, Sleep Deprivation & Regulation

Updated Dec. 14, 2020

Being fatigued does not necessarily mean feeling sleepy, though it can lead to that. The term “fatigue” describes a mental and physical state which can occur following a challenging or prolonged activity. A person who is fatigued has a lower-than-normal capacity for work and concentration while being less capable of completing any task efficiently.

Emotional stress, work (whether mental or physical) and sleep deprivation are all elements known to contribute to fatigue.

Common causes of fatigue

Practically every driver is exposed to factors and situations that cause fatigue on a regular basis. Check out some common causes of fatigue below. The more points on this list you can relate to, the greater the chances of fatigue rendering you unfit to drive safely.

  1. 1

    Sleep deprivation (through poor-quality sleep, insufficient time sleeping or too long a time since you last slept)

  2. 2

    Disruption of natural sleep cycles (as would be the case for people who work night shifts and sleep during the day)

  3. 3

    Prolonged periods of physical or mental work (such as studying, a long day at the office or physically strenuous activity)

  4. 4

    Stress (including general emotional stress, mental stress and stress caused by driving in an unfavorable situation – such as heavy traffic)

  5. 5

    Illness, injuries and disabilities (any condition which leads you to feel “under the weather” or causes prolonged discomfort)

  6. 6

    Medication (many common prescription drugs and over-the-counter medications are known to cause fatigue)

  7. 7

    Overeating (consuming a heavy meal can lead to feelings of fatigue and drowsiness, as your body works hard to digest the food)

  8. 8

    Sun glare (squinting or narrowing your eyes for a prolonged period)

  9. 9

    An overheated car (high temperatures can be exhausting and can lead to feeling sleepy)

The importance of sleep regulation

Even if your lifestyle does not allow you to keep an “ideal” sleeping pattern, it can be useful to understand how your body regulates sleep and what the effects this may have on how tired you feel throughout the day. In turn, this should help you to make a sensible and safe decision about when you should and should not drive.

There are two main processes that regulate sleep, known as sleep homeostasis and circadian rhythms. Find out more about these systems below.


Homeostasis is a chemical process that occurs in your body. Through this process, your body quietly maintains steady blood pressure, a healthy body temperature, normal acidity levels and crucially, causes you to “feel tired” when you need to sleep. Sleep is essential for recovery; without enough sleep, every system in the human body will cease to function as well as it should.

The average person needs about 8 hours of sleep for every 16 hours that they are awake. When we go beyond this 16-hour threshold, homeostasis causes us to feel increasingly tired the longer we remain awake. From the moment you wake up, the homeostasis clock is ticking. Eventually, you will reach a level of tiredness where staying awake becomes difficult.

Going to sleep at the end of the day (or at the end of your waking hours) does not necessarily reset the clock or clear all the “sleep debt” you accrued while awake. If you do not sleep long enough for the number of hours you were awake (I.e. roughly 8 hours for every 16 waking hours), your sleep debt will be carried over from the previous day. In this way, the effects of regular, insufficient sleep will accumulate until you feel tired constantly, have trouble thinking clearly and struggle to remain awake for an entire day. This situation will not resolve itself until you get extra sleep to make up for the hours you lost.

Circadian rhythms

Your hormone levels, body temperature and feelings of alertness will fluctuate throughout the day due to natural circadian rhythms. This process works in harmony with homeostasis to dictate your desire to sleep.

Your body’s circadian rhythms roughly follow the hours of darkness and light. You are more likely to feel wakeful and alert during daylight hours and should feel more inclined to sleep during the night. Circadian rhythms are not exact and do vary slightly from one person to another, however, most people experience an increased desire to sleep:

  • Between midnight and 6 am, and:
  • Between 1 pm and 3 pm.

Of course, certain lifestyle factors can influence to what degree you are affected by circadian rhythms. Caffeine consumption is one such factor. As both circadian rhythms and homeostasis stimulate sleep in most people at night, you are more likely to feel the effects of tiredness during this time.

Sleep regulation and driving impairment

The desire to sleep is the result of unavoidable physiological processes, which means you cannot “will” your way out of it. Stimulating substances such as caffeine can delay or lessen your need to sleep for a short time, though they should not be considered a worthy substitute for good-quality rest.

Attempting to drive when your body is craving sleep is dangerous and becomes more so the longer you remain awake. Eventually, the symptoms of fatigue will set in and impede your ability to detect hazards, react to danger, multitask and make logical decisions behind the wheel.

Keep in mind that any significant sleep deprivation or alteration to your usual sleeping schedule (perhaps due to jet lag or a late study session) may render you unfit to drive. You are responsible for making sure you can drive safely. If you find you are sleepy, emotional, easily distracted or having trouble concentrating, do not drive until you have caught up on some sleep.

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