Signs of dehydration during physical activity include increased thirst, dry mouth, fatigue, dizziness, and reduced performance. Athletes may also experience a decrease in motivation and concentration levels, as hydration is crucial for both physical and cognitive function. The body may start to show signs of reduced urination or darker urine color, indicating less fluid intake. If dehydration continues, symptoms can escalate to more serious issues like heat exhaustion or heat stroke. Therefore, regular hydration before, during, and after exercise is essential for maintaining performance and health.