Richard A. Friedman is a professor of clinical psychiatry at Weill Cornell Medical College.

Now that the country is slowly emerging from lockdown, we might consider what effect this vast experiment in social isolation has had on not just how we feel but also how we think.

Many of my friends and patients have been telling me they feel mentally duller and unfocused — even those who are still busy working and, in some cases, exercising even more than usual.

“I am tireder and dumber, failing to mark calendar things correctly,” said a friend who’s usually a very smart writer. A patient told me that every day feels the same as the next and he’s losing any sense of order. “What day is it, anyway?” he asked on a Zoom call.

I think they are on to something. Everyone knows that humans evolved as an intensely social species and that prolonged isolation can bring about anxiety, depression and insomnia and worsen many medical problems.

But isolation seems also to have negative effects on the brain and the ability to think. There is strong pre-covid-19 evidence that this is the case in people at all ages and in animals as well.

For example, we know that people with richer social networks and engagement have a reduced rate of cognitive decline over time. Lisa F. Berkman of the Harvard T.H. Chan School of Public Health analyzed data from a nationally representative sample of nearly 17,000 subjects age 50 and older from 1998 to 2004. Subjects were cognitively assessed with a simple word-recall test and then afterward at two-year intervals; meanwhile, social integration was measured by contact with family, friends and other social activities. The results showed that people with the highest level of social engagement had less than half the decline in their cognitive function of the least socially active subjects. Clearly, isolation is harmful to our brains.

And these effects of isolation can manifest quickly. A recent review of the psychological impact of previous quarantines found that health-care workers who were isolated for just nine days after possible exposure to SARS (another disease caused by a coronavirus) experienced poor concentration, indecisiveness and impaired work performance.

Besides depriving us of the company of friends, quarantine also deprives of us roaming around and exploring the outside world, with all its unexpected experiences and chance encounters. We are understimulated — and, as a result, many of us have experienced chronic low-level stress.

This might seem counterintuitive, since people usually think a lack of engagement — and the pressure that goes with it — can be pleasant and relaxing. But that’s not always true. Too little engagement can be just as stressful as too much. An extreme example is that of children in orphanages who are understimulated because of neglect, and suffer permanent emotional and intellectual deficits as a result.

Chronic stress is potentially harmful to the brain, in part because it leads to a persistent increase in the level of the stress hormones cortisol and norepinephrine. We know from animal and human studies that chronic elevation of cortisol can cause shrinkage of neurons in the hippocampus — a brain region that is critical to learning and memory.

Consider the study of a group of genetically identical mice who spent a few months in an enriched environment — a multitiered mouse house with lots of space and opportunity for roaming around.

Researchers discovered that the mice who spent more time exploring their new digs had significantly greater neurogenesis — the growth and development of neurons — in their hippocampus than their less adventurous mates. The implication is pretty clear: Being out and about and having new experiences can promote the growth of the hippocampus and be cognitively beneficial.

During the lockdown, many of us have tried to offset the problem of understimulation with virtual experiences such as Zoom and FaceTime. But all of us have been on a very short leash — stressed because we are isolated and understimulated, and thus perhaps not cognitively at our sharpest.

Fortunately, these cognitive deficits in adults appear to be reversible once the chronic stress subsides and you rejoin the outside world. The brains of children, though, are more neuroplastic, so for them chronic stress might have more long-term adverse effects.

As for me, I’ve missed the excitement of concerts and the movies during these long months. And though I never expected to say it, I even miss New York’s crowded subway cars and the adrenaline rush of crowds buzzing with energy. Who would ever have thought that such a noisy, jostling experience might have made me a sharper psychiatrist?

Watch Opinions videos:

Read more: