Does sleep affect IQ test performance?

Yes. Short-term sleep deprivation affects several cognitive functions. A meta-analysis by Lim and Dinges found the largest effects on simple attention and vigilance, with smaller but still measurable effects on complex tasks. Because reasoning tests depend on sustained attention, working memory and speed, poor sleep can lower scores, particularly on timed tests.

Sleep loss increases attention lapses: brief moments when processing drops out. In a test with many detailed items, even a few lapses can cause errors. Timed tests amplify the effect, because slower responses reduce the number of items completed. Complex reasoning is somewhat more resilient on average, perhaps because people invest extra effort, but that effort is harder to sustain across a whole test.

Two sittings of a similar online reasoning test, one after a normal night and one after four hours of sleep, can differ by more than people expect, largely because of careless errors and unfinished items in the tired attempt.