(CNN) — If you’re scheduling an appointment for a vaccination — whether for Covid-19, the flu or for travel to another country — make sure you’re getting a long, restful night’s sleep before you head to the doctor.

Sleeping less than six hours the night before you get the shot may limit your body’s response to the vaccine, reducing protection against the virus or bacteria, according to a new study.

“Good sleep not only amplifies but may also extend the duration of protection of the vaccine,” said senior author Eve Van Cauter, professor emeritus at the University of Chicago’s Department of Medicine, in a statement.

But there was one odd detail in the study’s findings: The impact of poor sleep on immune response to a vaccine was only scientifically relevant in men.

“Research that used objective measures of sleep deprivation, such as that of a sleep lab, found a decrease in the ability to respond to the vaccine that was particularly and statistically significant in males, but not females,” said study co-author Dr. Michael Irwin, distinguished professor of psychiatry and biobehavioral sciences at the UCLA Geffen School of Medicine.

Why would a man’s immunity be affected when a woman’s was not?

“There are known sex differences in immune response to foreign antigens, like viruses, and also to self antigens, like in autoimmune disorders, said Dr. Phyllis Zee, neurology professor and director of the Center for Circadian and Sleep Medicine at Northwestern University’s Feinberg School of Medicine.

“In general, women have stronger immune response, including (to the) flu vaccine,” said Zee, who was not involved in the study. “The evidence is that these differences reflect hormonal, genetic and environmental differences, which can change over the lifespan, so these differences may be less prominent among older adults.”

Regardless of your gender, if you’re sleep-deprived, jet-lagged, work a night shift or otherwise have swings in your sleep-wake cycle, consider delaying the vaccination, said Irwin, who directs UCLA’s Cousins Center for Psychoneuroimmunology and Mindful Awareness Research Center.

“If I was working with patients to give them a vaccination, I would inquire whether they’re having problems with sleep and whether they were sleep deprived the night before,” Irwin said. “If they are, I would ask them to come back when they are fully rested.”

What is good sleep?

The body needs to move through four stages of sleep several times a night. During the first and second stages, our bodies start to decrease their rhythms. Doing so prepares us for the third stage — a deep, slow-wave sleep where the body is restoring itself on a cellular level, fixing damage from the day’s wear and tear and consolidating memories into long-term storage.

Rapid eye movement sleep, also called REM sleep, is the final stage. Studies have shown that missing REM sleep, which is also when we dream, may lead to memory deficit and poor cognitive outcomes, as well as heart and other chronic diseases and early death.

On the flip side, years of research has found sleep — and especially the deepest, most healing kind — boosts immune response.

Most adults need seven to eight hours of relatively uninterrupted slumber to achieve restorative sleep, according to the US Centers for Disease Control and Prevention. Sleeping six or fewer hours a night — which many people do, especially during a busy work week — can cause a host of health problems.

Objective research showed association

The new study, published Monday in the journal Current Biology, undertook a meta-analysis on existing research on sleep and immune function after vaccination against influenza A and Hepatitis A and B.

When only studies that used self-reported sleep duration were analyzed, antibodies were reduced in people who slept less than six hours, but the association between a lack of sleep and immunity after vaccination was not scientifically significant.

However, when only studies that used objective measures — such as requiring people to come to a sleep lab, or when devices that can accurately track sleep were used — there was a “robust” association, especially for men, Irwin said.

One explanation for the difference between the findings of objective and self-reported research is that people typically overestimate the amount of sleep they get each night, the study said.

People who got less than six hours of sleep produced fewer antibodies than people who slept for seven hours or more, according to the analysis. The reduction in immune response affected adults between 18 and 60 more than people over the age of 65.

That wasn’t surprising, the statement said, because “older adults tend to sleep less in general, (so) going from seven hours of sleep per night to less than six hours is not as big of a change as going from eight hours to less than six.”

Immune response is the same

The study did not include analysis of antibody response to Covid-19 vaccines, because there are not yet adequate studies on sleep in Covid-vaccinated people, Irwin said. But he believes the results would still apply.

“How we stimulate the immune system is the same whether we’re using an mRNA vaccine for Covid-19, or an influenza, hepatitis, typhoid or pneumococcal vaccine,” Irwin said. “It’s a prototypical antibody or vaccine response, and that’s why we believe we can generalize to Covid.”

The team did perform an analysis which showed that, if a person arrived for a Covid-19 vaccination without adequate sleep, their antibody response to the vaccine would be weakened by the equivalent of two months — based entirely on their body’s initial response.

“You would have already lost two months of immunity, so to speak, even though you just got the shot,” Irwin said. “If you have a poor immune response, you are less likely to get full protection from Covid.”

More studies are needed to detect the nuances of poor sleep’s impact on the immune system, Zee said. Still, the information supports current practice in her sleep clinic.

“I already tell my patients to get regular sleep to enhance immune function,” she said. “Now we have even stronger evidence to give this type of advice.”