## 3 Results and Discussion
### Test Results of the \"TFK\" Sample
Beyond 60,000 cycles, the height change begins to decrease slightly, ending at 2.86 mm at 82,500 cycles. The initial increase in height suggests that mattress materials may expand or settle into a more stable configuration under cyclic loading. The height changes relative to the previous measurement fluctuated, indicating both increases and decreases in the height over the cycles. Notable changes occur at specific cycles (e.g., a decrease of \(-0.37\) mm at 37,500 cycles and \(-0.78\) mm at 82,500 cycles). After reaching a peak, a slight decrease in the height change indicated some level of material compression or settling. The largest negative change relative to the previous measurement suggests the beginning of the material fatigue or permanent deformation. The obtained results support the previous findings of similar research [35, 36, 37, 38, 39]--similar in terms of the assumption that the quality of the mattress affects the quality of sleep, which means that the next more comprehensive experiment could obtain results that will contribute to a better understanding of the relationship between long-term use of the mattress, changes in its structure and materials, and retention essential mechanical properties, leading to longer-lasting sleep comfort. For example, the hardness and firmness of a mattress significantly influence its comfort and support, which are critical for sleep quality and overall satisfaction [35]. The hardness value of polyurethane foam is a critical factor influencing the firmness rating of mattresses, both essential for optimizing mattress performance. Hardness (assessed through indentation force/load deflection), quantifies the resistance of the foam to compression. A higher hardness value typically correlates with a firmer mattress. Practically, this means that it ensures less immersion and envelopment, which, for example, can affect the prevention of pressure ulcers. Conversely, lower hardness values indicate softer foams that allow for greater immersion and envelopment, potentially enhancing comfort and pressure relief. That relationship between hardness and firmness is essential for optimizing mattress performance [36, 37]. A well-balanced firmness level is essential to achieve optimal comfort and support in mattress design. Investigating the objective and subjective comfort of mattresses with different hardnesses, [PERSON] et al. [35] identified optimal hardness values that correlate with improved comfort, suggesting that the hardness rating of pocket spring mattresses should be aligned with the resulting hardness recommendations in order to improve the quality of sleep. The study by [PERSON] et al. [38] highlights that the structural properties of spring mattresses, including the bedding materials and mattress core, significantly affect sleep quality. An optimal combination of these elements can enhance mattress comfort, leading to improved sleep depth and quality. [PERSON] et al. [39] show in their study that the hardness of pocket spring mattresses directly affects their firmness rating, which in turn affects overall comfort and support. According to their findings, the hardness of a mattress is related to its ability to conform to the body's natural curves, ensuring proper spinal alignment and reducing pressure points, thereby improving sleep quality and comfort ratings. Over time, mattresses undergo changes in hardness due to compression, wear, and material degradation. Changes in mattress firmness over prolonged use can compromise supportive properties and sleep comfort. This study aimed to investigate the possible effects of long-term mattress use on firmness and sleep quality, but indirectly, by investigating changes in mattress properties. Understanding how changes in mattress hardness over time might influence sleep quality can have significant implications for both sleep science and mattress design industries.