Objectives: The discrepancy between the intention to go to bed and actual behavior is called bedtime procrastination (BP), which is associated with sleep deprivation and reduced sleep time; the higher the level, the higher the daytime sleepiness and fatigue. This study examined how fear of missing out (FoMO), social network service (SNS) addiction proneness, and negative affect (NA) impact BP. Methods: In total, 273 adults (male, 46.5%; female, 53.5%; mean age, 27.15±1.86 years) completed the FoMO scale, SNS addiction proneness scale, NA scale, and BP scale. Results: First, only proneness to SNS addiction had a significant effect on BP (B=0.228, <i>p</i><0.01). This study verified the significance of the indirect effect and found that SNS addiction proneness completely mediated the relationship between FoMO and BP (B=0.086, 95% CI=0.023– 0.152). Second, the moderating effect of NA was significant in the relationship between FoMO and SNS addiction proneness (B=0.126, <i>p</i><0.01). Third, the moderated mediating effect of NA through SNS addiction proneness was significant in the relationship between FoMO and BP (Moderated Mediation Index=0.029, 95% bootstrap CI=0.004–0.060). Conclusions: This study is significant in that it verifies the relationship between FoMO and BP, SNS addiction proneness, and NA, and identifies motivational and behavioral factors that cause BP.