What effect does the "midnight sun" have on people?

Varying degrees of insomnia, differing as to duration and onset, and even depression may affect the people who experience the midnight sun phenomenon.

"The midnight sun is a phenomenon occurring in latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous 24 hours, mostly north of the Arctic Circle and south of the Antarctic Circle. The number of days per year with potential midnight sun increases the further poleward one goes."
https://en.wikipedia.org/wiki/Midnight_sun

"Many find it difficult to fall asleep during the night when the sun is shining. In general, visitors and newcomers are most affected. Some natives are also affected, but in general to a lesser degree. The effect of the polar night, that is, not experiencing day for long durations of time is said to cause Seasonal Affective Disorder or S.A.D., a type of depression caused by lack of natural light. High-intensity daylight-spectrum lamps and tanning beds are a popular solution for relief from S.A.D."
https://en.wikipedia.org/wiki/Midnight_sun#Effect_on_people

However, this sleeplessness and depression is not as bad as that which accompanies the "dark period" (midwinter insomnia) when the sun does not rise above the horizon.
https://link.springer.com/chapter/10.1007%2F0-387-23692-9_28

This problem is shown in the movie "Insomnia".

NOTE: Throughout the film, Dormer becomes more and more unhinged, partly because of feelings of guilt over shooting his partner and anxiety about the risk of being found out, partly because of insomnia brought on by the perpetual daylight of the Alaskan summer.

https://en.wikipedia.org/wiki/Insomnia_(2002_film)

Some more info:

Although most intriguing, the midnight sun has been shown to have taxing effects on the human body and it would be expected that these effects would be worse in a body unaccustomed to these seasonal extremeties. Mental disorders have been shown to increase in severity and peaks in suicide figures have been demonstrated during the summer months. Local winters, when there is a significant lack of sunlight, can trigger depression and those suffering from seasonal affective disorder should consider their susceptability to these effects.

[From article at aber.ac.uk/biology, no longer online]

For some strange reason it seems like the people from Iceland suffer much less from SAD related symptoms than people from the rest of Scandinavia. As a Norwegian I definitively suffer from it.

https://en.wikipedia.org/wiki/Seasonal_affective_disorder