Sleep inertia (the grogginess feeling that one feels immediately after waking) is usually expected and not an issue for most of the population as waking and acting immediately is not a common requirement. For those working in settings that require this fast acting upon waking (eg: an on-call doctor or a military personnel), the amount of sleep they achieved prior can greatly affect their response time.
In a study performed by McHill et al., the relationship between total sleep time and sleep inertia were looked at. A total of 26 participants were enrolled in the study and placed in a group where total sleep time was capped at 5.6 hours or have a minimum of 8 hours. Once awoken, participants were issued the Digit Symbol Substitution Task (DSST), a task that measures working memory and processing speed, and the Karolinska Sleepiness Scale (KSS), a questionnaire that identifies how sleepy someone is.
Results showed that those who slept less had a performance that was 10% worse than those who had the recommended 8 hours of sleep and remained impaired across the dissipation of their sleep inertia. For the ordinary person, this is not significant to their daily functioning but for those who are immediately required to act upon awakening, the total sleep time is crucial to them for performing to the best of their ability.
Full study: https://academic.oup.com/sleep/article/42/5/zsz032/5307024
