Vertical grooves or ridges are a normal part of aging and are likely caused by cell turnover within the nail.
Horizontal grooves or ridges are not normal and may indicate a health problem. Horizontal indentations and ridges of the nail itself, called Beau's lines, are believed to be caused by a cessation of cell division due to illness or injury.
Skin diseases and infections such as pruritis can cause horizontal grooves, as well as medical conditions such as uncontrolled diabetes, circulatory diseases such as coronary occlusion, and hypocalcaemia. High fever associated with illnesses such as mumps, measles, or pneumonia can also cause Beau's lines.
Certain drugs may also be responsible for Beau's lines such as chemotherapy treatments as well as malnutrition.