How do you feel when you haven’t slept well? People often feel irritable, anxious, short-tempered, and stressed. How well do you function when you haven’t slept well? Most people have trouble sustaining their attention, understanding new information, and remembering what they’ve learned. You may be thinking that you perform better than that, but another feature of poor sleep is that people have inaccurate perceptions of their own abilities¹—in other words, you think you’re doing better than you really are.

Learn how your brain functions (or doesn’t) when you’re sleep-deprived, and why you should never make important decisions on a poor night’s sleep!

Sleep deprivation may increase reactivity of the amygdala

Sleeping facilitates communication between two general areas of the brain, the primitive brain and the executive brain.² The primitive brain governs basic survival (biological drives, the fight-or-flight response), memories, and simple value judgements. It also includes the fear processing part of the brain, the amygdala. Since the information processed through the primitive brain is related to survival, the decisions and feelings associated with this information occur very quickly—often before you consciously realize how or why you have reacted. The types of decisions made through the primitive brain are often snap judgements, and the feelings tend to be polarized (e.g., like-dislike, disgust-attraction). 

The executive brain includes the part of the brain called the prefrontal cortex. This area is responsible for rational decision-making and complex emotions. Because it integrates information from multiple areas of the brain, processing information through the advanced brain takes more time than with the primitive brain. Although it takes more time to arrive at decisions and feelings, the reactions from the executive brain tend to better reflect the complexity of a given scenario. In other words, decisions and feelings tend to be more appropriate when processed through the executive brain. 

The executive brain receives information from the primitive brain about safety, rewards, and memories, and combines that information with inputs from other parts of the brain to consciously decide how to act. However, when we’re sleep-deprived, the communication between the primitive and executive brains is significantly compromised. Brain imaging studies have shown that sleep deprivation increases the reactivity of the amygdala (the fear processing part) by 60%.^3,4 Further, connectivity between the amygdala and the prefrontal cortex (the rational part of the brain) is diminished, which can have a tremendous impact on how we think.²

Lack of sleep causes us to think with our primitive brain affecting our decision making

When communication between executive and primitive brains is compromised, as with sleep deprivation, we are essentially thinking with the primitive brain. This causes us to overvalue perceived “rewards” and undervalue actual negative consequences. As a result, people tend to make decisions that they otherwise wouldn’t. For example, a person might:²

• Inflate the actual value of monetary rewards
• Disregard financial losses
• Enhance the actual value of cheap or free food 
• Choose high calorie foods and/or overeat

So, don’t wait to find yourself writing checks at a get-rich-quick seminar, irritably eating your fourth free doughnut before you realize that you need better sleep.

As sleep loss accumulates, dominance of the amygdala drives a negative psychological state.² This means that you essentially become a more negative version of yourself—perhaps more pessimistic, impatient, or judgmental. At the same time, activities that would normally enhance your mood become less effective.² In short, the longer you go without sleep, the more likely you are to experience the world in terms of missed opportunities and unsatisfying rewards. 

Sleep to forget, sleep to remember

As described in our article “What Happens When We Sleep?”, a lot of neuronal activity occurs in many parts of the brain during REM sleep. This is especially true of the parts of the brain that govern emotion. While the neurons in the emotional brain are active during REM sleep, the amount of the neurotransmitter norepinephrine they contain decreases significantly. In fact, norepinephrine reaches its lowest levels (of any time during the day or night) during REM sleep. 

Why would norepinephrine levels drop so dramatically during REM sleep? One of the functions of norepinephrine is to “tag” memories with emotions.⁵ This function is highly beneficial for remembering positive and negative experiences, because it helps us to seek out or avoid those experiences in the future. Remember the time you volunteered at the food bank and it felt really good? You’ll do that again. Remember the time you were sleep-deprived and ate cheap shrimp sold from the back of a van and it felt really bad? You won’t do that again. However, like most biological processes in the body, this function of norepinephrine can be problematic if left uncontrolled, such as when memories are so emotionally charged that they become disabling. 

It’s believed that the reduction in norepinephrine during REM sleep may be a control mechanism for preventing the maladaptive emotional charge of memories.² By decreasing norepinephrine, REM sleep after traumatic events can diminish the emotional strength of the experience, while still preserving the memory of it. This preserves the adaptive function of “tagging” memories with emotions by retaining the information of the memory, while minimizing the dysfunctional part of the memory.

This is called the Sleep to Forget, Sleep to Remember (SFSR) model of REM sleep, and it has its roots in how we process learned fears.² Incredibly, our ability to discriminate between fears from actual threats (e.g., a gunshot) versus conditioned threats (e.g., a door slamming) is predicted by the amount of REM sleep immediately after the traumatic event. So, the more sleep you get after a traumatic event, the better able you are to differentiate between fears and actual threats. This has powerful implications for how mental health and medical professionals address the immediate needs of trauma sufferers at risk for post-traumatic stress disorder.

Try to get a good night’s sleep

It’s easy to disregard what happens to your mind when you don’t sleep if you can just “push through it”. In our society, sleep deprivation for the sake of work and social obligations is more acceptable (and expected) than the critical mental healthcare that sleep provides. You deserve what happens when we sleep: Authentic emotions, rational decision-making, and the reduction of fears that keep us from being our best.