Acknowledging the critical role of spinal alignment during sleep in sleep quality, this work investigates the influence of mattress firmness on spinal alignment across different age groups and BMI categories, aiming to contribute to more personalised mattress recommendations in the long run. This study involves 150 healthy participants, aged 20–60, who were asked to lie on 3 different mattresses categorized as soft, firm and medium. Spinal alignment was assessed using Kinovea software to analyze the angles at the shoulder, torso and hip zones. Statistical analysis was performed to determine the impact of mattress type, age and BMI on spinal alignment. Results indicated that generally, the overall spinal deformation is significantly different among different mattress firmness levels, specifically in the shoulder and torso zone. The findings revealed that age and BMI significantly influenced the spinal alignment, particularly on soft and firm mattress. Medium mattress offers the most consistent spinal support across different age and BMI groups. This research emphasizes the importance of selecting a mattress based on individual age and BMI to achieve optimal spinal alignment and improve sleep quality.