The use of fans during sleep can affect individuals with pre-existing respiratory conditions, such as asthma or allergies, by circulating allergens in the air. This may lead to symptoms like coughing, wheezing, or nasal congestion. Additionally, the concentrated airflow can cause muscle stiffness or cramping, particularly in the neck, and may dry out the skin and eyes, worsening conditions like eczema or dry eye. While these effects are generally not severe or long-lasting, they can cause discomfort and disrupt sleep quality, especially for those sensitive to allergens or prone to dryness.