There are some days when you feel like you could workout forever, and then there are those days where you’re tired after 15 minutes. The days where you have to will yourself to finish are frustrating, but your body may be telling you something. Here are some of the most common reasons why your workout feels harder than it should on some days.

You’re Not Drinking Enough

If you’re struggling to get through your workout, you may not be drinking enough fluid. Exercise performance starts to suffer when you’ve lost as little as 2% of your body water. When that happens, you’ll start to feel tired and less motivated to exercise. When you reach a level of 5% dehydration, you can really feel the effects. Don’t count on thirst to tell you when you need to drink. That usually comes after you’ve already lost too much fluid. Keep a water bottle handy during a workout, and sip between 5 and 10 ounces every 15 minutes while you’re exercising, and drink 15-20 ounces before starting to workout. If you’re planning a session that lasts longer than an hour, sip a sports drink that contains electrolytes.

You’re Not Minding Your Nutrition

If you’re feeling drained during an intense workout, you may be glycogen depleted. This is more likely to happen if you do a high-intensity workout first thing in the morning. During high-intensity workouts, your body uses primarily glycogen as a fuel source, and if you work out intensely, your glycogen stores may hit rock bottom after 1 to 2 hours. Not only will you feel fatigued, but you’ll have to slow down. You’re less likely to feel overly tired if you’re work out a moderate intensity since your body primarily burns fat at a lower intensity.

If you do a vigorous workout, have a snack with about 30 to 40 grams of carbs before exercising. If you’re in a rush a piece of fruit or energy bar will work. If you plan on working out for more than an hour, bring along a sports drink.

You’re Overtraining

If you’re regularly feeling fatigued during exercise or having trouble getting through your usual workout and you’re clocking a lot of hours at the gym, you may be overtraining. Take a few days off to let your body rest and recover. Other signs of overtraining including an increased resting heart rate, lack of appetite, weight loss, muscle soreness, difficulty recovering from exercise, increased susceptibility to colds and flu, changes in sleep patterns and loss of enthusiasm for working out. If you have these symptoms, you may need a longer period of rest.

You Have an Undiagnosed Medical Problem

A number of medical problems including diabetes and an underactive thyroid can make a workout feel harder. One of the more common causes, particularly among females, is iron deficiency anemia. If fatigue is a persistent problem, see your doctor.

You’re Not Getting Enough Sleep

Your workout may seem harder than usual because you’re not getting enough sleep. Make it your goal to get at least 7 hours of sleep a night. When you don’t sleep enough, exercise will feel more challenging, and your risk of catching colds and flu bugs at the gym because your resistance to infection will be lower.

You’re Bored

Sometimes boredom can make exercise seem harder because your nervous system is rebelling against doing the same workout over and over again. Don’t let tedium cause you to lose your enthusiasm and turn exercise into a drudgery. Try a new routine, and get a new burst of enthusiasm. It’ll also help you break out of a plateau.