There may be several causes
for it, which include excessive worry, stress, fear, anxiety
, pain, grief, aging, mental disorders like depression, post-traumatic stress disorder, and generalized anxiety disorder. Sometimes other medical (overactive thyroid gland) or neurological disorders might lead to insomnia.
If the condition is transient, it should resolve on its own .However, if it persists then you need to see your doctor for a complete evaluation.