Yes, using an AC at night is safe for a 6-month-old, provided the temperature is set to a comfortable level (typically 24-25°C). Ensure the baby’s crib is placed away from direct airflow and that they are dressed in appropriate layers. Regular servicing of the AC filters is recommended to minimize allergens, and using a humidifier can prevent the air from becoming too dry. If you are worried, keep a window slightly ajar to allow for fresh air circulation.