Nocturnal cramps. Night time cramps in the calf muscles are common, and often there is no evident cause. They occur more commonly with advancing age (and during pregnancy). Occasionally, they can be caused by other conditions, such as diabetes, electrolyte imbalance (eg, from kidney disorder, or from taking a diuretic water pill) or other metabolic conditions. Your doctor can evaluate you for any of these causes.
Tired muscles. Cramping is what you are describing and the causes include prolonged standing/working on legs, venous incompetence which results in swelling, excessive exercise, abnormalities in the blood potassium, calcium, magnesium. See a phlebologist, vein doctor who can evaluate your legs.