What time of day actually changes in older adults' thinking
In older adults, time of day has been linked to how sleepiness and alertness relate to performance. Among 133 older adults, greater daytime sleepiness was associated with poorer immediate learning and memory in the afternoon but not in the morning, and lower current arousal was associated with worse processing speed in the afternoon [1]. These are task-specific patterns, not a general statement that all thinking is better or worse at a given hour.
Other work points in the same direction: cognitive performance can fluctuate across the day, and in older adults with mild cognitive impairment, morning versus afternoon testing conditions influenced working memory performance [4]. A study of the Mini-Mental State Examination in people with Alzheimer's disease and age-associated cognitive decline also examined time-of-day effects, and its authors raised the idea that time of day might affect more fluid abilities such as attention [5]. That was a hypothesis about what might be happening, not a measured comparison of fluid and crystallized abilities.
Why fluid and crystallized abilities might respond differently
Fluid abilities are the on-the-spot mental operations — processing speed, attention, working memory, and learning something new. Crystallized abilities are accumulated knowledge and vocabulary, which tend to be more stable. The tasks that showed time-of-day effects in these studies — immediate learning and memory, processing speed, working memory — are closer to the fluid side [1][4]. That is consistent with the idea that fluid abilities are more vulnerable to daily fluctuations, but it is not a direct test, because the studies did not also measure crystallized performance at the same times.
The Mini-Mental State Examination study explicitly framed its expectation around fluid versus crystallized intelligence, suggesting that time of day would affect more fluid cognitive abilities such as attention [5]. Again, this is a proposed expectation rather than a demonstrated contrast between the two ability types.
The same hour is not the same for everyone
Time-of-day effects are not uniform across people. A study of middle and older adults using repeated smartphone-based cognitive assessments across the day aimed to test whether daily cycles in cognitive performance emerge and whether people differ in those cycles, with those individual differences then examined against global cognitive functioning and psychosocial factors [2]. This matters for the fluid-versus-crystallized question because an average time-of-day effect could hide different patterns in different people.
There is also evidence that circadian and wake-dependent influences on sleepiness, cognitive throughput, and reaction time can differ between older and younger adults [3]. That study compared age groups on reaction time and cognitive throughput at various times of day, but it did not separate fluid from crystallized abilities, and its authors noted uncertainty about whether the findings apply to more typical older adults or to different types of cognitive performance [3].
What this means if you are comparing abilities or scheduling assessments
The practical takeaway is that time of day is worth accounting for when assessing cognition in older adults, especially for tasks involving learning, memory, processing speed, and working memory [1][4]. But there is no evidence here that lets you say fluid abilities are affected by time of day while crystallized abilities are not, or vice versa. The studies that came closest to the question treated the fluid-versus-crystallized distinction as an expectation to be tested rather than a result [5], and the studies that measured time-of-day effects used tasks that do not cleanly separate the two [1][4].
A direct answer would require measuring both kinds of ability in the same older adults at multiple times of day, and ideally tracking individual differences in daily rhythms rather than relying on group averages [2]. Until that is done, the honest answer is that the difference between fluid and crystallized abilities remains an open question, not a settled finding.
Sources used in this answer
Time of Day Effects in the Relationships Between Daytime Sleepiness, Current Arousal, and Cognitive Performance
Among 133 older adults, time of testing moderated the link between daytime sleepiness and immediate learning/memory (poorer performance in the afternoon but not morning) and between current arousal and processing speed (lower arousal linked to worse afternoon performance).
INDIVIDUAL DIFFERENCES IN TIME OF DAY EFFECTS ON AMBULATORY COGNITIVE FUNCTIONING IN MIDDLE AND OLDER ADULTS
A study of 200 middle and older adults using five daily smartphone surveys for 20 days aimed to test whether intra-day cycles in cognitive performance emerge and whether individual differences in those cycles relate to global cognitive functioning and psychosocial factors.
Circadian and wake-dependent influences on subjective sleepiness, cognitive throughput, and reaction time performance in older and young adults
Circadian and wake-dependent influences on subjective sleepiness, cognitive throughput, and reaction time were examined in older and young adults at various times of day, with the authors noting uncertainty about applying the findings to more typical older adults or different cognitive performance types.
The mixed role of sleep and time of day in working memory performance of older adults with mild cognitive impairment
In older adults with mild cognitive impairment, morning versus afternoon testing conditions influenced working memory performance, showing that time of day affects cognitive tasks.
Exploring Time of Day Effects on Mini-Mental State Examination Performance in Alzheimer's Disease and Age-Associated Cognitive Decline
Time of day affected Mini-Mental State Examination performance in people with Alzheimer's disease and age-associated cognitive decline, and the authors raised the expectation that time of day would affect more fluid abilities such as attention.