A baby crying after a short sleep may be caused by calcium deficiency, noisy environment, improper daily diet, illness, and other factors. Here are some possible reasons:

1. Calcium Deficiency

Poor sleep quality in babies is generally caused by calcium deficiency. Babies should normally start taking vitamin AD drops around 15 days after birth, with a daily dose of 400 units of vitamin D once a day, usually until around 18 months of age. When the weather is good, parents should take their babies out to get some sun for about two hours a day to promote calcium absorption. Additionally, parents should consider feeding and ensure that the baby is fed well and has a clean diaper before bedtime to ensure a restful sleep.

2. Noisy Environment

The environment is crucial for newborns, and most babies prefer a quiet setting. Sleeping in an excessively noisy environment can cause the baby to wake up frequently. Therefore, it is best to provide a peaceful sleep environment for the baby to ensure quality sleep.

3. Improper Daily Diet

Many parents intentionally introduce new foods to their babies to promote their health. However, unsuitable foods can severely impact the baby's sleep, causing them to wake up frequently. Therefore, parents should be cautious about introducing new foods and ensure that the baby's diet is suitable for their age and health.

4. Illness

Illnesses can also cause babies to cry during sleep. If parents notice that their child frequently wakes up and cries after a short sleep, it is advisable to seek medical attention promptly to ensure the baby's health and well-being.