Millions of Americans were lulled into a false sense of security during the pandemic that they had enough money to retire. Roughly 2 million people retired early, according to the Federal Reserve.

Retiring without knowing whether you have enough money to cover your day-to-day expenses for 20 to 30 years or even longer—let alone medical costs, which keep rising—is one of the “worst money mistakes” you could make, Harvard PhD economist Laurence Kotlikoff tells Yahoo! Finance.

Although the stock market plummeted at the very start of the pandemic, the government’s trillions of dollars in COVID stimulus drove up equities and 401(k) accounts in the two years that followed.

Plus, remote working gave people the freedom to work from home and a taste of breaking away from the office.

People in their 50s or early 60s began to think it was time to retire, Kotlikoff says.

Bad mistake, he says, as many retirees have learned in the past year, witnessing extreme market volatility deplete their retirement savings.

The fact is, Kotlikoff maintains, people are terrible savers to begin with. On top of that, he adds, they fail to project near-term budgetary needs, let alone decades into retirement.

“We are, as a group, lousy savers—making early retirement unaffordable,” Kotlikoff says. “Financially speaking, it’s generally far safer and far smarter to retire later.”

How far behind are Americans on retirement savings?

Very. There’s a mere $65,000 in the median American retirement account. Those between the ages of 55 and 64 have just $134,000 saved.

To put in perspective just how far below the mark this is, Fidelity Investments projects that a 65-year-old couple retiring in 2022 will need $315,000 to cover out-of-pocket costs throughout their retirement years. That doesn’t even take into consideration the possibility of needing long-term care.

Retiring early not sacrifices not just earning power and larger retirement nest eggs but also bigger Social Security benefits. A new study suggests that most Americans should wait until they're 70 or older to begin claiming Social Security benefits.

The National Bureau of Economic Research study found that Social Security beneficiaries are missing out on receiving nearly $200,000 by claiming too early.

Like many other financial planners—especially in light of inflation—Kotlikoff recommends working for as long as you are capable of doing so and want to.

So, don’t hang it up too early, the Boston University economist says. If you do retire too early, he says, it could be an expensive mistake that you’ll soon regret.