The child doesn't sleep during the day and also goes to bed very late at night.

Children who do not sleep during the day and also go to bed very late at night may be experiencing this due to insufficient daytime activity, lack of vitamin D, hyperthyroidism, or other causes. Treatment should be based on the specific underlying condition.

1. Insufficient daytime activity: If a child engages in little physical exercise for prolonged periods, reduced physical activity can lead to not napping during the day and going to bed very late at night. In such cases, parents should increase the child's outdoor activity time. After moderate physical activity, the child will feel tired, which promotes sleep and helps correct the pattern of no daytime naps and late nighttime sleeping.

2. Vitamin D deficiency: When children do not consume enough vitamin D in their daily diet or lack sufficient exposure to outdoor sunlight over a long period, their nervous system may become overly excitable, resulting in no daytime sleep and very late bedtimes. Parents should promptly supplement the child’s vitamin D intake and spend more time outdoors with the child to promote the body's synthesis of vitamin D and calcium.

3. Hyperthyroidism: Children may develop hyperthyroidism due to genetic factors, thyroiditis, or other causes, leading to increased sympathetic nervous system activity, which disrupts sleep and results in failure to nap during the day and extremely late bedtimes. In such cases, prompt medical attention is necessary, and treatments such as antithyroid medications—including methimazole tablets or propylthiouracil tablets—should be administered under medical supervision.

A child's habit of not sleeping during the day and staying up very late at night may also stem from personal routines. Parents should supervise and regulate the child's daily schedule, helping them establish healthy habits of going to bed early and waking up early.