Sleep is the most important thing that humans need. Going all the way back to the first lineage of human-like species, sleep would have created a state of vulnerability and thus those who slept would have been naturally selected against, but it was absolutely imperative for their bodies to repair and replenish themselves overnight. Sleep is a highly conserved behaviour across animal evolution, which demonstrates its importance for survival and fitness.

Sleep is characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity, as well as the inhibition of nearly all voluntary muscles during deep sleep (John Hopkins Medicine, n.d.). There is a decreased overall reactivity to stimuli, but those who are asleep still show unique and active brain signals. During sleep, most of the body’s systems are in an anabolic state which helps to restore the immune, nervous, muscular, and skeletal systems; these are extremely vital processes that help to maintain a healthy mood, memory, and cognitive functions, and also plays are large role in ensuring proper functions of the endocrine and immune systems (John Hopkins Medicine, n.d.).

The internal circadian rhythm promotes daily sleep at night. This is our “internal clock” that recognizes the difference between day and night and ensures that our bodies are well rested at night in order to function properly throughout the day. The internal circadian rhythm is what causes individuals to become “jet-lagged” when they travel to different time-zones. Humans’ need for sleep varies depending on the individual; sleep is considered to be adequate when there is no daytime sleepiness or dysfunction (John Hopkins Medicine, n.d.). This would indicate that the body has fully replenished overnight and is able to function properly throughout the day. Age also plays a role in how much sleep individuals need on a given night. Younger children require more sleep in order to develop and function properly: up to 18 hours for newborns, with the rate decreasing as the child ages. The recommended amount of sleep for adults is between 7 to 9 hours per night (John Hopkins Medicine, n.d.).

There are several factors that can disturb the balance of our circadian rhythm. One common factor is blue light exposure. Sources of blue light include the sun, digital screens, and LED light. Blue light suppresses the secretion of melatonin, a hormone that promotes sleepiness and shifts circadian rhythms (Harvard News Letter, 2018).

Inconsistent sleep schedules are another factor that disrupts sleep. The body is unable to establish a circadian clock, which leads to difficulty in falling asleep (Kang & Chen, 2009). Stress is also a factor that alters the sleep-wake cycle. Stress triggers the autonomic nervous system to release hormones, such as adrenaline and cortisol. These hormones are responsible for a reaction known as the flight-or-fight response, which raises one’s alertness and promotes arousal. Frequently being in a heightened state of alertness can delay the onset of sleep and adversely affect one’s sleep quality (Johnson, 2018).

Many would assume exercising before bed will also negatively impact sleep quality as it is another activity that stimulates cortisol release. However, scientists found that vigorous late-night exercise has no association with sleep and does not affect sleep quality (Myllymäki, et al., 2011). The effect of caffeine on sleep is another controversial discussion. Over the years, researches showed contradicting findings. One study suggested that consumption of caffeine 6 hours before bedtime has a significant disruptive effect on sleep. On the other hand, another study examined the relationship between caffeine, alcohol, nicotine, and sleep. Nicotine and alcohol use within 4 hours of bedtime was associated with increased sleep fragmentation, but caffeine was not associated with any of the sleep parameters (Spadola, et al., 2019). The effect of caffeine appears to vary among individuals.

Last but not least, eating near bedtime may also undermine sleep. While some food, such as warm milk and chicken breast, may help us relax and contain chemicals that promote sleep, spicy and acidic food like citrus and tomatoes can induce heartburn, which can lead to insomnia (Peters, 2019).

Animal and human studies have shown that the quantity and quality of sleep play a critical role in learning and memory. Sleep helps learning and memory by allowing an individual to focus their attention maximally throughout the day, thereby allowing efficient learning to occur (Fowler, 1973). Sleep also has a profound effect on memory consolidation, which is the process in our brain used to convert short term memory to long term memory (Stickgold, 2005). It is recommended that students get at least 6 hours of sleep for tests and exams in order to retain and recall information. This also partially explains why all-nighters are not recommended to students. Memory consolidation takes place during sleep through the strengthening of the neural connections that form our memories (Stickgold, 2005). During sleep, the brain organizes information and classifies information depending on what we need and what we don’t need (Sleep, Learning, and Memory, 2020).

Sleep is critical for learning as it contributes to the three essential stages of learning: Acquisition, Consolidation, and Recall, in which each of these steps is necessary for proper memory function (Graves, 2001). Acquisition refers to the introduction of new information into the brain. Consolidation represents the processes by which a memory becomes stable. Recall refers to the ability to access the information after it has been stored. Although acquisition and recalling of information occur when an individual is awake, the consolidation, which links the acquisition stage (i.e. learning of new information) to recalling stage (being able to remember and apply the information), occurs when an individual is asleep (Graves, 2001). In other words, recalling of information cannot occur efficiently nor effectively without a proper memory consolidation. Thus, sleep plays a critical role in learning, memory, and concentration.