During the brain’s activity when we are awake, potentially toxic waste products are produced and accumulated in the brain.¹ For healthy brain functioning and mental health, these waste products need to be cleared out.¹ Such clear-out happens much faster during sleep, making sleep an essential contributor to healthy brain functioning.¹

Emotions
You must have heard the phrase “someone woke up on the wrong side of the bed” when referring to someone who is in a bad mood. This expression nicely illustrates how not having enough quality sleep impacts on our emotional well-being, which most people have experienced themselves. The relationship between sleep and emotions has been confirmed by research as well, showing that lack of sleep causes increased negative affect, where individuals experience more frequent and more intense negative (like anger, fear) than positive emotions (happiness) when they don’t get enough sleep.²
 
Not only does sleep deficiency lead to negative emotions, but it also affects emotional memory formation negatively, meaning that negative memories are more preserved than positive ones, leading to further negative feelings.³
 
There is also evidence that lack of sleep impairs emotional control in a way that sleep deprived individuals process neutral stimuli the same way as negative stimuli, which hinders the accurate discrimination of emotions.⁴
 
Cognition
Prolonged sleep restriction has pronounced detrimental effects on certain aspects of cognition as well³ which most people are familiar with. The effects of sleep deprivation are comparable to that of alcohol intoxication (0.05% blood alcohol concentration), where response speed slows down, accuracy decreases, and performance on divided attention tasks declines.⁵ Such impairments start appearing as early as 17-18 hours after waking up.⁵
 
Further studies have shown that not getting enough sleep markedly impairs attention, where reaction time slows down and vigilance is impaired.³ Sleep restriction negatively impacts memory as well, both during (memory formation) and after (memory consolidation) learning.³
 
Anxiety and depression
Experiencing marked negative emotions as a result of sleep restriction can lead to mental health problems.⁶ For instance, sleep deprivation was positively associated with anxiety symptoms and distress in healthy individuals.⁷ Even after one night of sleep deprivation, anxiety and distress levels increased, while those who slept as normal reported decreased distress and anxiety.⁷ In the longer term, lack of sleep might contribute to the development of generalised anxiety disorder.⁷
 
In addition to anxiety, the link between depression and sleep problems is well-established: studies found that individuals with insomnia have more than two times higher risk of developing depression than those who do not suffer from insomnia.⁶ However, in case of short-term sleep deprivation, research suggests that it might be helpful for those suffering from depression.⁸ Studies found that acute, or even partial (second half of the night) sleep deprivation for a single night had the potential to enhance mood for the following day in around 60% of depressed individuals.⁸ However, most individuals relapsed in mood following the next night of sleep.⁸ To maintain the positive effects of short-term sleep deprivation, adding light therapy, or drugs to sleep deprivation were found to be effective in preventing relapse.⁸ So, although short-term sleep deprivation holds promise for improving depressive symptoms, the evidence is not straightforward yet.
 
Psychosis
Even in healthy individuals, insomnia was found to be associated with the development of hallucinatory experiences.⁹ Those suffering from mild insomnia had a two-three-fold risk of experiencing hallucinations, while those suffering from chronic insomnia had a four-fold risk of developing hallucinatory experiences 18 months later, linking insomnia directly to hallucinations.⁹
 
Therefore, a good night’s sleep is key!