The main reasons why a cough often gets worse at night include the body's circadian rhythm, changes in hormone levels such as cortisol, lying down, and factors like postnasal drip and airway sensitivity. At night, immune cells become more active causing inflammation to fight infection, which can increase symptoms. Cortisol levels drop at night, reducing inflammation suppression and allowing symptoms to worsen. When lying down, mucus can pool in the throat due to gravity, triggering coughing. Additionally, night-time coughing can be worsened by allergies, asthma, acid reflux, or respiratory infections, which tend to have heightened effects during the night. More specifically:

• The body's internal clock increases immune cell activity near bedtime, which triggers inflammation and worsens symptoms like coughing.
• Lower nighttime cortisol allows more inflammation than during the day.
• Lying flat allows mucus from nasal passages to drip to the throat, stimulating cough reflex.
• Nighttime factors like cooler air, lower humidity, and increased airway sensitivity contribute to worsened coughing.
• Conditions such as asthma, acid reflux (GERD), and allergies can cause or aggravate coughing at night.
• Sleeping with head elevated or on the side can help reduce pooling of mucus and acid reflux, thus minimizing cough.

Overall, the cough does not necessarily get worse because the illness is stronger at night, but because of physiological and positional factors that make symptoms more noticeable and irritating during sleep.