The basic effect: people perform best at their preferred time of day
Chronotype is a person's natural sleep-wake leaning: morning types rise and peak early, evening types peak later. The "synchrony effect" is the tendency for each type to do better on cognitive tasks at the time of day that matches that leaning. A 2026 study comparing morning and evening types on verbal (word-based) and visuospatial (spatial-arrangement) tasks found differences in performance across the day, consistent with this idea [2]. A 2025 systematic review of the chronotype literature likewise concludes that chronotype does modulate cognitive performance, though it notes the extent of that modulation remains unclear [3].
The effect is not uniform across everything a person does. A 2018 study of working memory recognition tasks found time-of-day effects on response bias and false-alarm rates, meaning people's willingness to say "yes, I saw that" shifted with the clock, not just their raw accuracy [1]. That distinction matters: time of day may change how cautious or trigger-happy someone is in judging what they remember, which is a different thing from being smarter or duller.
Does a harder task make the time-of-day effect bigger?
Intuitively, a demanding task should expose a tired brain more than an easy one, so you might expect the chronotype effect to swell as difficulty rises. The direct evidence for that is mixed and limited. The 2018 working memory study reported no interference of task complexity with circadian rhythmicity, which argues against difficulty being a strong amplifier, at least for the recognition tasks it examined [1]. The 2026 verbal and visuospatial study looked at interactions between time of day and task difficulty, but its abstract does not establish that difficulty reliably changed the size of the chronotype effect [2].
The systematic review takes the broader view that task complexity is one of the external factors that can affect arousal and performance alongside chronotype [3]. Read carefully, that is a claim that complexity belongs on the list of relevant influences, not a demonstration that it systematically strengthens or weakens the morning-versus-evening gap. So the honest answer is: plausible, widely assumed, and not yet pinned down by studies that vary difficulty within the same people across the day.
Does lighting change the picture, and for whom?
Light is a known lever on alertness, so it is a reasonable candidate for shifting the chronotype effect. A 2019 study of indoor illuminance (how bright a room is) and correlated color temperature (whether light looks warmer or cooler) found relationships between lighting, positive mood, and cognitive performance during daytime office hours, and it compared morning with afternoon periods [4]. That shows lighting and time of day can interact in ordinary daytime work, but the study did not compare morning types with evening types, so it cannot tell us whether bright or cool light helps evening types more at their off-peak hours.
A 2026 study of daylight in educational interiors goes further in framing, examining how time of day, task complexity, and lighting jointly relate to cognitive performance while taking chronotype into account [5]. Its focus on daylight illuminance and individual characteristics suggests the relationship may be non-linear, meaning more light is not simply better and the effect may depend on who is in the room. That is a promising direction, but it is a framing and design claim rather than a settled result about how much lighting rescales the morning-evening gap.
What this means if you are scheduling your own hard thinking
The safest practical reading is to protect your peak window for demanding work and treat lighting as a secondary dial rather than a fix. The synchrony effect itself is reasonably well supported across verbal and visuospatial tasks [2] and across the broader literature [3], and it appears to touch judgment and response style as well as raw accuracy [1]. Lighting during the day is linked to mood and performance [4], and daylight in particular may relate to performance in ways that depend on the individual and are not simply linear [5].
What is not established is the size of the adjustment: no study here shows that making a task harder, or brightening a room, reliably widens or narrows the morning-versus-evening performance gap by a measurable amount. If you are an evening type facing a morning deadline, the evidence supports expecting some mismatch and some benefit from good lighting, but not a confident prediction of how much either factor will move your performance.
Sources used in this answer
Would you say “yes” in the evening? Time-of-day effect on response bias in four types of working memory recognition tasks
Time of day affected response bias and false-alarm rates in working memory recognition tasks, and task complexity did not interfere with circadian rhythmicity in these tasks.
Hemispheric efficiency during the day: the influence of circadian preference and time-of-day on verbal and visuospatial tasks
Morning and evening types differed in verbal and visuospatial task performance across the day under constant lighting and temperature, though the abstract does not establish that task difficulty reliably changed the chronotype effect.
Chronotype and synchrony effects in human cognitive performance: A systematic review
Chronotype modulates cognitive performance, though the extent remains unclear, and external factors such as task complexity and lighting conditions can affect arousal and performance.
Effects of illuminance and correlated color temperature on daytime cognitive performance, subjective mood, and alertness in healthy adults
Indoor illuminance and correlated color temperature were related to positive mood and cognitive performance during daytime office hours, comparing morning and afternoon periods but not morning versus evening types.
Non-linear Relationships Between Daylight Illuminance, Individual User Characteristics, and Cognitive Performance in an Educational Interiors.
Daylight illuminance, individual characteristics including chronotype, time of day, and task complexity appear to relate to cognitive performance in educational interiors in potentially non-linear ways.