It's one thing to experience passing moments of stress because of bills or student loans. It's another thing to be stressed out over finances all the time. But that's the reality for about a third of Americans, according to a recent survey from mobile-banking company Varo Money.

Varo polled more than 1,000 U.S. adults and found that, while a whopping 85 percent say they "sometimes" feel stressed about money, a full 30 percent say they're "constantly" stressed about their finances.

Why? Well, 66 percent of adults, including 71 percent of millennials, say it's because they don't have a three-month emergency fund, and 46 percent say it's because they don't have any savings set aside in one to cover an unexpected expense like a job loss or medical problem.

Other studies have made similar points. Less than half of Americans have enough to cover a $1,000 emergency, according to a survey from financial website Bankrate, and in a similar survey financial site GOBankingRates found those aged between 18 and 24 had less than $1,000 in their savings accounts, period. Nearly half have nothing saved at all.

That could explain why 43 percent of Americans label "unexpected expenses" as their biggest concern.