Though SWS is fairly consistent within the individual, it can vary across individuals. Age and gender have been noted as two of the biggest factors that affect this period of sleep. Aging is inversely proportional to the amount of SWS beginning by midlife and therefore, SWS declines with age. Sex differences have also been found, such that females tend to have higher levels of SWS compared to males, at least up until menopause. There have also been studies that have shown differences between races. The results showed that there was a lower percentage of SWS in African Americans compared to Caucasians, but since there are many influencing factors (. body mass index, sleep-disordered breathing, obesity, diabetes, and hypertension) this potential difference must be investigated further. 
The primary genetic factor that has been strongly implicated in the development of narcolepsy involves an area of chromosome 6 known as the human leukocyte antigen (HLA) complex.   Specific variations in HLA genes are strongly correlated with the presence of narcolepsy;  however, these variations are not required for the condition to occur and sometimes occur in individuals without narcolepsy.   These genetic variations in the HLA complex are thought to increase the risk of an auto-immune response to orexin -releasing neurons in the lateral hypothalamus .   
The other placebo study included 20 knees in the treatment group and 18 knees in the placebo-treatment group. The third study provided a comparison between patients treated with three weekly injections of Hyalgan® followed by 2 weekly treatments with arthrocentesis with patients treated with arthrocentesis for five weeks, and arthrocentesis and placebo injections for five weeks. Additional arms of this study assessed additional treatment regimens. Statistical evaluation of the data was performed at day 60. In this study, only patients considered to be success were followed beyond day 60. These patients were followed for 180 days, however, due to the number of dropouts, statistical evaluation was not performed on data gathered at time points beyond day 60. The results of these investigations reported that the three-injection Hyalgan® treated patients experienced pain relief beginning at day 21 and continuing throughout the remaining 60-day observation period.