This is an observational study for the assessment of normal overnight variation in small healthy children using the Ventica Lung Function Testing System. The study is carried out to establish reference ranges for overnight lung function variation using the Ventica test, and to assess the impact of the demographic factors on the Ventica results.

Inclusion Criteria:

• 18 months-6 years, both sexes
• Healthy at the time of inclusion
• Laboratory analyses in the reference range or not clinically relevant according to the investigator judgement
• Lung function in the reference range with a negative bronchodilator test result
• Signed informed consent

Exclusion Criteria:

• Preterm birth
• Chronic respiratory disorder of prematurity
• Recurrent bronchitis or recurrent hospitalizations because of a respiratory illness (recurrent pneumonia)
• Primary immunodeficiency
• Chronic rhinosinusitis, synonasal polyposis
• Nasal congestion, adenotonsillar hypertrophy, signs and symptoms of sleep apnea or sleep disordered breathing
• Acute respiratory infection or hospitalization because of an acute illness within 4 weeks prior to inclusion
• Personal or family history or clinical evidence of asthma, atopy or other chronic respiratory disorders (high risk for asthma and allergy)
• Use of asthma rescue or maintenance medication within 4 weeks prior to inclusion
• Implanted or external active medical devices such as pacemakers

Young children screened to have small probability of asthma or wheezing