Large-scale research points to a U-shaped relationship with the lowest risk around seven hours of sleep per night, but this is a population average, not a personal target. The association with long sleep is difficult to interpret causally, because illness itself can cause a greater need for sleep. How rested you feel during the day is a more reliable measure than counting hours.
The large analysis behind this finding looked at tens of thousands of people and found a U-shaped relationship: both short and long sleep are associated with a higher risk of earlier death and cardiovascular problems, with the lowest risk at around seven hours per night. That number is, however, a population average, not a personal target. Your own optimum may fall somewhere between six and nine hours, depending on age, genetics and health.
An important thing to understand: the association with long sleep in particular is difficult to interpret. People who are chronically ill often sleep more because the body demands it. The longer sleep duration is then a consequence, not a cause, of the higher risk. Short sleep and its associated risks are somewhat more firmly supported as a genuine, causal direction, but here too science is not yet entirely certain.
What you can practically take from this: if you structurally sleep five hours or fewer, or regularly need nine hours or more for no clear reason, that is worth paying attention to -- not necessarily as an alarm bell, but as a signal to look into whether something is going on with your sleep quality or underlying health. Counting hours is less valuable than noticing how rested you feel during the day. If you regularly wake up tired despite enough hours, or find you need a lot of sleep without explanation, that is a reason to discuss with a doctor whether your sleep quality may be disrupted, for example by apnoea.
Moderate evidence, 1 source(s); the direction is likely but not conclusively proven.