I attended a Thanksgiving dinner several years ago where the hostess, without warning family and friends, broke with tradition and served salmon instead of turkey, roasted potatoes instead of mashed, raspberry coulis instead of cranberry sauce and … you get the idea.

While a few guests mustered the politesse to say the meal was “something else,” most reacted with undisguised dismay. Some seethed. Others sulked. One young guest actually cried. No one had seconds.

It wasn’t that the meal itself was bad. In fact, the meal was outstanding. The problem was that it wasn’t the meal everyone was expecting.

When there are discrepancies between expectations and reality, all kinds of distress signals go off in the brain. It doesn’t matter if it’s a holiday ritual or more mundane habit like how you tie your shoes; if you can’t do it the way you normally do it, you’re biologically engineered to get upset.

This in part explains people’s grief and longing for the routines that were the background melodies of their lives before the pandemic — and also their sense of unease as we enter a holiday season unlike any other. The good news is that much of what we miss about our routines and customs, and what makes them beneficial to us as a species, has more to do with their comforting regularity than the actual behaviors. The key to coping during this, or any, time of upheaval is to quickly establish new routines so that, even if the world is uncertain, there are still things you can count on.

First, a little background on why we are such creatures of habit. Psychologists, anthropologists, neuroscientists and neurobiologists have written countless books and research papers on the topic but it all boils down to this: Human beings are prediction machines.

“Our brains are statistical organs that are built simply to predict what will happen next,” said Karl Friston, a professor of neuroscience at University College London. In other words, we have evolved to minimize surprise.

This makes sense because, in prehistoric times, faulty predictions could lead to some very unpleasant surprises — like a tiger eating you or sinking in quicksand. So-called prediction errors (like finding salmon instead of turkey on your plate on Thanksgiving) send us into a tizzy because our brains interpret them as a potential threat. Routines, rituals and habits arise from the primitive part of our brains telling us, “Keep doing what you’ve been doing, because you did it before, and you didn’t die.”

So the unvarying way you shower and shave in the morning, how you always queue up for a latte before work and put your latte to the left of your laptop before checking your email are all essentially subconscious efforts to make your world more predictable, orderly and safe.

Same goes for Tuesday yoga class, Friday date night, Sunday church services, monthly book clubs and annual holidays. We may associate these activities with achieving a goal — health, friendship, education, spiritual growth — but the unwavering regularity and ritualized way with which we go about them, even down to our tendency to stake out the same spot in yoga class or sit in the same pew at church, speak to our need to minimize surprise and exert control.

Routines and rituals also conserve precious brainpower. It turns out our brains are incredibly greedy when it comes to energy consumption, sucking up 20 percent of calories while accounting for only 2 percent of overall body weight. When our routines are disrupted, we have to make new predictions about the world — gather information, consider options and make choices. And that has a significant metabolic cost.

Dr. Friston said that our brains, when uncertain, can become like overheated computers: “The amount of updating you have to do in the face of new evidence scores the complexity of your processing, and that can be measured in joules or blood flow or temperature of your brain.” That exertion, combined with the primordial sense of threat, produces negative emotions like fear, anxiety, hopelessness, apprehension, anger, irritability and stress. Hello, Covid-19.

Our brains are literally overburdened with all the uncertainty caused by the pandemic. Not only is there the seeming capriciousness of the virus, but we no longer have the routines that served as the familiar scaffolding of our lives. Things we had already figured out and relegated to the brain’s autopilot function — going to work, visiting the gym, taking the kids to school, meeting friends for dinner, grocery shopping — now require serious thought and risk analysis.

As a result, we have less bandwidth available for higher order thinking: recognizing subtleties, resolving contradictions, developing creative ideas and even finding joy and meaning in life.

“It’s counterintuitive because we think of meaning in life as coming from these grandiose experiences,” said Samantha Heintzelman, an assistant professor of psychology at Rutgers University in Newark who studies the connection between routine behavior and happiness. “But it’s mundane routines that give us structure to help us pare things down and better navigate the world, which helps us make sense of things and feel that life has meaning.”

Of course, you can always take routines and rituals too far, such as the extremely controlled and repetitive behaviors indicative of addiction, obsessive-compulsive disorder and various eating disorders. In the coronavirus era, people may resort to obsessive cleaning, hoarding toilet paper, stockpiling food or neurotically wearing masks when driving alone in their cars. On the other end of the spectrum are those who stubbornly adhere to their old routines because stopping feels more threatening than the virus.