Thank you for your query,
Your child seems to have Anxiety Neurosis which is common in adlescent girls.
Because of anxiety heart rate will increase, because of increased syphathetic outflow associated with anxiety and during sleep and in quiet periods the heart rate will be low corresponding to her base level of heartbeat.
Evaluation for hyperthyroidism
is important as it will present with similar complaints.
She requires counselling
and anxiolytic medications like alprazolam ,so consult your Pediatrician
or child psychiatrist
for further management.
Hope i have answered your query.