The early signs are obvious: you'll feel thirsty and tired. If you keep going, you may get a headache, muscle cramps, or start to feel nauseous. If you notice your urine is a darker color than normal, either during or after your run, it definitely means you need haven't been hydrating enough to make up for the water lost through sweat. Even when drinking properly, it's normal to end up a bit dehydrated at the end of a run on a warm day. Make sure to drink a few glasses of water once you're back home to get your body back to a balanced state after your run.