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Why is it harder to get through your workouts some days?

woman exersizing

Most days, you’re killing it at barre or crushing your run. But sometimes, your usual HIIT class feels like a total struggle and you’re not sure why. It’s not that your instructor is tricking you with tougher moves, or that time is slowing down so your treadmill session feels endless. It’s likely a direct result of something else you’re doing—or not doing. What you eat and drink, how often you exercise, how much quality sleep you’re getting...all of those directly impact how easy (or hard) breaking a sweat will feel. So if those 8-pound weights all of a sudden feel like 80 pounds, one of the reasons below may be to blame.

You’re Dehydrated

If you feel like every rep is a struggle, think back to the last time you had some water. The average adult drinks about 39 ounces per day, according to the Centers for Disease Control and Prevention  (CDC)—not quite the 91 to 125 ounces recommended by the Food and Nutrition Board.

When the body is severely dehydrated, you don’t have enough water to sweat,” explains Asheesh Gupta, M.D, a sports medicine physician with the Centers for Advanced Orthopaedics. “Because sweat is the primary way to decrease body temperature, the lack of water can cause overheating, lower blood pressure, and possibly fainting. Dehydration can also lead to elevated heart rate, cramping, loss of endurance, fatigue, electrolyte imbalance, and sometimes feelings of lethargy.” Frankly, none of that is going to making working out feel good.

woman eating bar on the go

You’re Not Fueling the Right Way

Everyone loves to hate on carbs, but carbs are your body’s major fuel source, says Isabel Maples, R.D.N. “Cutting carbs back too much can make you feel sluggish,” she explains. “The body stores quick energy, called glycogen, right in the muscles for easy access. It's also stored in the liver, where the body can mobilize it for that hard workout.” If those stores aren’t there, well, there’s nothing for your body to burn.

And that doesn’t just mean fueling up pre-workout. “The body seems to be better able to replenish glycogen stores right after an exhaustive workout, which means that eating after a hard workout is a good idea,” Maples says. “Some people have a hard time eating right away—they might be busy or maybe they're not hungry, especially if they're dehydrated.” But she says that research suggests that as long as you’re eating within two hours or so, that’s OK.

You’re Not Getting Enough Sleep

Most of the benefits of exercise actually come after you leave the gym. That’s why getting enough shuteye is so important. When you sleep, your body uses that time to recover, conserve energy, and repair and build up the muscles you worked. “What you’re doing in a class or workout is causing an acute level of damage that places a demand on your body to adapt and recover,” explains Eric Bowling, NASM-certified personal trainer at Ultimate Performance Los Angeles. “You change and grow when you rest, not during the session itself.” If you’re not getting enough sleep, “the body can’t effectively adapt and recover, which can make you feel even more exhausted.”

Not getting enough sleep can also make your workout seem tougher, according to one review of research published in the journal Sports Medicine. In other words, it may not affect your physical capabilities, but you could feel tired faster, which can make that 45-minute spin class feel like it’s lasting 45 years. A more recent review of research concluded that more studies need to be done on this topic, pointing out that the reasons why this happens won’t be understood without more info (after all, your grumpy mood from being tired may be a factor). On the flip side, getting enough sleep may potentially motivate you to follow through on your workout plans, according to one study of people with insomnia published in the Journal of Clinical Sleep Medicine

woman stopping mid run

Your Immune System Isn’t at Its Best

First things first: Exercise itself isn’t likely to weaken your immune system, according to research published in Frontiers in Immunology; in fact, regular exercise is a good idea for your immune health. But if you’re already sick, exercise is a stressor that may cause your already beleaguered system to suffer. “The same mechanisms that are used to fight illness also help you to recover from controlled training stress,” Bowling explains. “If you put too much demand on this system, then something has to give.”

One of the best things you can do to help your immune system is to get enough good quality sleep, Bowling says. While extra sleep may not prevent you from getting sick, the National Sleep Foundation agrees that insufficient sleep may have negative effects on your immune health. To help support your immune system with nutrients, take Emergen-C Immune+ vitamin drink mix, which is packed with key vitamins C and D, plus electrolytes. And don’t forget other healthy habits, like eating a healthy diet and managing your stress. If you’re totally wiped, don’t push yourself through an intense workout. Opt for a lower-intensity workout like restorative yoga, or take a rest day—you deserve it.

You’re Overtraining

If you feel like you’re plateauing at your favorite workout—or if it’s actually getting harder—you might be training too much. “Overtraining actually increases an athlete’s chances of injury because you’re using the same muscle groups all the time,” Dr. Gupta says. And once you’re injured, it’s going to be that much harder to reach your goals.

It can also affect you psychologically, he adds. Athletes who are overtraining typically don't have the same enthusiasm or desire to work out, and they don’t recover as well after their workouts.” Dr. Gupta recommends two to three rest days per week if you’re working out four to five days per week. But that doesn't just mean lounging around on your couch. Practice active recovery:Yoga and meditation can help to increase blood circulation, decrease muscle soreness, and help with overall stress levels,” he says. “Resting your mind is just as important as resting your body.