Common causes include insufficient or poor-quality sleep, stress, and blood-sugar swings from irregular eating. Other contributors are anemia (including iron deficiency), low B12/folate, thyroid disorders, infections, chronic inflammation, depression/anxiety, dehydration, and certain medications. If symptoms are new and impair daily life, check for red flags (chest pain, shortness of breath, fainting, palpitations, weight loss, fever, night sweats, bleeding). Without red flags, improving sleep, nutrition, and activity for a few weeks—plus targeted labs—can be a practical first step; persistent or worsening fatigue needs clinical assessment.