Sleeping fewer than seven hours a night roughly triples your odds of catching a cold compared to sleeping eight hours or more, making sleep one of the most powerful and underused defenses against respiratory viruses. But sleep is only part of the picture. A handful of other everyday habits, from managing stress to keeping indoor air humid, can meaningfully shift the odds in your favor. Some popular supplements earn their reputation, while others fall flat once you look past the marketing. The evidence behind each strategy varies widely, and a few common practices actually carry risks that rarely get mentioned.
Sleep Is the Single Biggest Lever You Control
Two well-designed studies, both of which deliberately exposed healthy volunteers to cold viruses and then tracked who got sick, tell a remarkably consistent story. In the first, people averaging fewer than seven hours of sleep per night were about three times more likely to develop a cold than those getting eight hours or more, even after accounting for other health habits.1PubMed Central. Sleep habits and susceptibility to the common cold The second study used wrist-worn activity monitors to measure sleep objectively rather than relying on self-reports. People sleeping fewer than six hours had more than four times the odds of developing a cold compared to those getting seven or more hours.2PubMed Central. Behaviorally Assessed Sleep and Susceptibility to the Common Cold The relationship was graded: the less you slept, the more vulnerable you became.
What makes these studies especially convincing is their design. Researchers squirted live rhinovirus into participants’ noses and then quarantined them for several days, so everyone received the same viral dose. The difference in who got sick came down to the host, not the exposure. Sleep appears to affect how quickly and effectively your immune system mounts an early inflammatory response, the kind that clears a virus before symptoms ever develop. If you’re trying to avoid a cold and only change one thing, sleeping at least seven hours is the strongest single move the evidence supports.
Zinc Helps Shorten Colds More Than It Prevents Them
Zinc is one of the most studied supplements for the common cold, but the evidence splits sharply depending on whether you’re talking about prevention or treatment. A 2024 Cochrane review pooling nine prevention trials found that zinc supplements produced little to no reduction in the risk of catching a cold.3PubMed Central. Zinc for prevention and treatment of the common cold That’s a disappointment for anyone hoping a daily zinc tablet would keep them healthy through winter.
Treatment is a different story. The same Cochrane review found that zinc taken after symptoms start may cut the duration of a cold by roughly two days, though the evidence was rated low-certainty and the variation between studies was enormous.3PubMed Central. Zinc for prevention and treatment of the common cold An earlier meta-analysis reported a similar shortening in adults, with zinc lozenges reducing cold duration by about a day and a half on average, but found no meaningful effect in children.4PubMed Central. Zinc for the treatment of the common cold: a systematic review and meta-analysis of randomized controlled trials A third analysis focused specifically on lozenge formulations found that colds were about a third shorter in zinc groups overall, with zinc acetate lozenges outperforming zinc gluconate.5PubMed Central. Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage
The practical takeaway: keeping zinc lozenges on hand for the first sign of a scratchy throat is reasonable. Taking zinc daily all winter to avoid colds in the first place probably won’t do much. And whatever you do, avoid intranasal zinc products. Zinc gel and spray formulations applied directly inside the nose have been linked to a specific kind of permanent smell loss. The pattern is distinctive: people spray, sniff, feel a burn, and then lose their sense of smell, sometimes permanently.6PubMed. Intranasal zinc and anosmia: the zinc-induced anosmia syndrome Zinc ions are directly toxic to the olfactory tissue that detects odors.7American Journal of Rhinology. Anosmia after Intranasal Zinc Gluconate Use Oral lozenges do not carry this risk.
Vitamin C Mostly Disappoints, With One Exception
Vitamin C is the most famous cold remedy in the public imagination, and the most overhyped. A Cochrane review covering more than 10,000 participants found that regular vitamin C supplementation did not reduce how often people in the general population caught colds.8PubMed Central. Vitamin C for preventing and treating the common cold That’s a definitive result across a very large number of trials.
The exception involves people under heavy physical stress. Among marathon runners, skiers, and soldiers training in extreme cold, vitamin C supplementation roughly halved the incidence of colds.8PubMed Central. Vitamin C for preventing and treating the common cold A separate review focused specifically on this population confirmed the effect at doses between 600 milligrams and one gram daily.9PubMed. Vitamin C and common cold incidence: a review of studies with subjects under heavy physical stress If you’re training for an ultra-marathon or doing extended outdoor military exercises, vitamin C may be worth taking. If you work a desk job, it won’t prevent colds, though it may modestly shorten them by a small fraction of a day.
Vitamin D Is Not the Shield People Think
Vitamin D supplements are widely marketed for immune support, and earlier research suggested they might reduce respiratory infections. But a large, updated meta-analysis of randomized trials paints a more complicated picture. Looking across all participants regardless of their starting vitamin D levels, vitamin D showed no clear protective effect against respiratory infections for any baseline group.10The Lancet Diabetes & Endocrinology. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials
That said, the picture shifts when you look at how the vitamin D was given. An earlier version of this analysis found that daily dosing at moderate levels (roughly 400 to 1,000 IU per day) did show a protective effect, while intermittent large bolus doses did not.11PubMed Central. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials This suggests that daily, moderate supplementation might help some people, but that the mega-dose vitamin D capsules sold in many pharmacies are probably not useful for cold prevention. If you’re already taking a moderate daily vitamin D supplement for bone health, there’s no strong reason to stop based on respiratory concerns, but there’s also no strong evidence to start one purely to dodge colds.
Stress Makes You Measurably More Vulnerable
Psychological stress is one of the most underappreciated risk factors for catching a cold. A landmark study published in the New England Journal of Medicine exposed healthy volunteers to cold viruses and found that the rate of developing a clinical cold climbed steadily with self-reported stress levels, from about 27 percent in the least-stressed group to 47 percent in the most-stressed group.12PubMed. Psychological stress and susceptibility to the common cold That’s nearly double the cold rate between the calmest and most stressed people, receiving the same virus.
Later research explored the mechanism. Chronic stress appears to make immune cells less responsive to cortisol, the hormone that normally keeps inflammation in check. When that feedback loop breaks down, inflammation runs unchecked, and the body’s ability to fight off a new infection suffers. People showing this pattern of cortisol resistance were at higher risk of developing colds and, once infected, produced more of the inflammatory chemicals that drive cold symptoms like congestion and sore throat.13PubMed Central. Chronic stress, glucocorticoid receptor resistance, inflammation, and disease risk In short, stress doesn’t just make you feel worse when you’re sick. It makes you more likely to get sick in the first place.
Moderate Exercise Helps, But Overdoing It Backfires
Regular moderate exercise appears to support immune function and reduce the frequency of upper respiratory infections. The relationship follows a J-shaped curve: sedentary people get sick more than moderately active people, but very intense or prolonged exercise temporarily suppresses immune defenses and increases infection risk.14PubMed Central. Sport, exercise, and the common cold This matches the vitamin C findings above, since the populations where vitamin C actually worked were precisely those under extreme physical stress from heavy training.
For most people, the practical advice is straightforward: regular walks, cycling, swimming, or other moderate activity is good for cold resistance. If you’re training for competitive endurance events, recognize that the training load itself can create a vulnerability window, and pay extra attention to sleep, nutrition, and recovery during heavy training blocks.
Hand Washing and Masks Still Work
Cold viruses spread through respiratory droplets and contaminated surfaces. Hand washing remains one of the simplest and most effective barriers. A systematic review of community-based studies found that frequent hand washing was associated with a significantly lower likelihood of catching influenza, with children washing five or more times daily showing about 40 percent lower odds of laboratory-confirmed infection.15PubMed Central. Effectiveness of hand hygiene practices in preventing influenza virus infection in the community setting: A systematic review Interestingly, alcohol-based hand sanitizer did not show a clear benefit in the same analysis, possibly because people use it less thoroughly than soap and water.
As for masks, a Cochrane review of physical interventions to interrupt respiratory virus spread found that surgical masks were among the most consistently supported measures.16PubMed Central. Physical interventions to interrupt or reduce the spread of respiratory viruses N95 respirators performed about the same as standard surgical masks for general respiratory virus protection, which suggests that for everyday cold avoidance, a basic surgical mask during peak cold season on a crowded train or plane is a reasonable option if you’re willing to wear one.
Dry Air Is Your Nose’s Worst Enemy
Humidity is an overlooked factor in cold susceptibility. Your nasal passages are lined with a mucus layer that physically traps and clears viruses before they can infect cells. That clearance system depends heavily on moisture. Dry indoor air, the kind produced by winter heating, slows mucus movement, impairs the immune cells stationed in the airway lining, and makes symptoms worse once an infection takes hold.17PubMed. Indoor air humidity revisited: Impact on acute symptoms, work productivity, and risk of influenza and COVID-19 infection Low relative humidity changes the thickness and flow properties of nasal mucus, essentially gumming up the escalator that would otherwise carry viruses out of your airway.18PubMed Central. Relative Humidity and Its Impact on the Immune System and Infections
Keeping indoor humidity in the 40 to 60 percent range during winter helps preserve that mucosal defense. A simple hygrometer and a humidifier can do the job. Staying hydrated matters too: one small study found that drinking a carbohydrate-electrolyte beverage before entering a dry environment helped maintain faster nasal mucus clearance compared to no prehydration, though water alone didn’t show the same effect.19PubMed. Effect of prehydration on nasal mucociliary clearance in low relative humidity
Why Cold Air Itself Weakens Your Defenses
There’s a reason cold viruses love the nose. Rhinoviruses and common cold coronaviruses replicate best at the temperature of the nasal cavity, around 33°C, which is several degrees cooler than core body temperature. Research in mouse airway cells showed that at full body temperature, cells mounted a far stronger antiviral response through interferon signaling, the alarm system that rallies neighboring cells to resist infection. At the cooler nasal temperature, that alarm was muted, giving the virus a replication advantage.20PubMed Central. Temperature-dependent innate defense against the common cold virus limits viral replication at warm temperature in mouse airway cells A 2024 study in human nasal cells confirmed this pattern, finding that common cold viruses preferentially replicate at 33°C, where they trigger weaker interferon responses than they do at 37°C.21PubMed Central. Interferon signaling in the nasal epithelium distinguishes among lethal and common cold coronaviruses and mediates viral clearance
This doesn’t mean you’ll catch a cold every time you step outside in winter, but it does mean that breathing cold, dry air actively weakens the front-line defenses in your nose. Wrapping a scarf over your nose and mouth on frigid days isn’t just about comfort; it warms and humidifies the air you breathe, which gives your nasal immune system better working conditions.
Carrageenan Nasal Sprays Show Early Promise
Iota-carrageenan is a seaweed-derived compound that forms a physical gel over the nasal lining. Several clinical trials have tested carrageenan nasal sprays as a barrier against cold viruses, and the results are cautiously promising. In one trial involving children, the carrageenan group cleared their illness about two days faster than the placebo group, had lower viral loads, and developed fewer secondary infections with other respiratory viruses.22PubMed Central. Lessons learned from a double-blind randomised placebo-controlled study with a iota-carrageenan nasal spray as medical device in children with acute symptoms of common cold A review of multiple trials found consistent trends toward reduced viral loads in the carrageenan groups across studies.23PubMed Central. Safety and efficacy of iota-carrageenan nasal spray in treatment and prevention of the common cold
These sprays are available over the counter in many countries. They’re not a guaranteed shield, and the trials so far have been relatively small. But carrageenan is inert, non-medicated, and has no known serious side effects, which puts it in a different safety category than the zinc nasal products discussed earlier. Think of it as a physical barrier rather than a drug.
Probiotics, Elderberry, and the Placebo Effect
The gut-lung axis is an active area of research, and there’s growing evidence that the bacteria in your gut influence respiratory immunity. Clinical studies suggest that probiotic supplements can support immune function across age groups.24PubMed Central. Potential probiotics for regulation of the gut-lung axis to prevent or alleviate influenza in vulnerable populations Some specific probiotic strains appear to modulate immune activity in the lungs through this gut-lung connection, which could help the body respond to respiratory viruses.25PubMed. Probiotics as a biotherapeutics for the management and prevention of respiratory tract diseases The evidence here is promising but still developing; no one can point to a specific probiotic product and say with confidence that it will prevent colds.
Elderberry is another popular option. In a trial of air travelers, those taking elderberry extract didn’t catch significantly fewer colds, but when they did get sick, their colds were about two days shorter and significantly less severe than in the placebo group.26PubMed Central. Elderberry Supplementation Reduces Cold Duration and Symptoms in Air-Travellers: A Randomized, Double-Blind Placebo-Controlled Clinical Trial Lab studies have shown that elderberry compounds can interfere with influenza virus replication, particularly at later stages of the viral life cycle.27Journal of Functional Foods. Anti-influenza activity of elderberry (Sambucus nigra) Like zinc, elderberry seems to be more useful once you’re already sick than as a daily preventive.
And then there’s the placebo effect, which turns out to be genuinely relevant for the common cold. In a controlled trial, participants who received pills and believed in their effectiveness had shorter, less severe colds, regardless of whether the pills actually contained anything active.28PubMed Central. Placebo effects and the common cold: a randomized controlled trial This doesn’t mean colds are “all in your head,” but it does suggest that the immune response to a cold is partly influenced by psychological state, which loops back to the stress findings discussed earlier. If taking a supplement gives you a genuine sense of confidence that you’re doing something proactive, that psychological boost may not be entirely empty.
Why Winter Brings More Colds Than Just Cold Air
Most people assume they catch more colds in winter because cold weather weakens their immune system. The temperature effect on nasal cells is real, as covered above, but seasonality is more complicated than that. An analysis of viral challenge studies found that day length itself was significantly associated with the probability that an infection progressed to a symptomatic cold: shorter days correlated with higher susceptibility.29iScience. Susceptibility to the common cold virus is associated with day length This was independent of actual temperature and held even in controlled laboratory settings where participants were kept at constant indoor temperatures.
Day length probably acts as a proxy for several overlapping factors. Shorter days reduce sun exposure and shift circadian rhythms, both of which influence immune cell activity. People also spend more time indoors in winter, in closer proximity to others and in drier heated air. It’s not a single cause. Winter brings together a cluster of conditions, less sleep, more stress around the holidays, drier air, colder nasal passages, reduced daylight, more crowded indoor spaces, that collectively tilt the odds toward infection.
Why Older Adults Get Sicker, and What That Means for Prevention
Age changes the landscape of nasal immunity. Research on human nasal tissue has found a striking decline in immune cell presence in the nasal lining with age, particularly monocytes, a type of immune cell that serves as a first responder to infection.30PubMed Central. Immune cell residency in the nasal mucosa may partially explain respiratory disease severity across the age range Meanwhile, the structural epithelial cells that line the airway don’t decline the same way, meaning the physical barrier is intact but the immune surveillance behind it is thinner.
Separate work on nasal epithelial cells from older versus younger adults found that key immune signaling molecules, the ones responsible for presenting viral fragments to the rest of the immune system, were expressed at lower levels in older cells, both at baseline and during active infection.31The Journals of Gerontology: Series A. Age-Associated Changes in the Respiratory Epithelial Response to Influenza Infection This helps explain why older adults are more susceptible to respiratory infections and why their colds tend to drag on longer. For people over 65, the prevention strategies outlined here, adequate sleep, hand hygiene, humidity management, moderate exercise, aren’t just nice-to-haves. They’re compensating for a genuinely diminished first line of defense.