Cognitive performance — particularly alertness varies throughout the day based upon the time of day, the degree of interest in a task, and the degree of sleepiness not to mention the degree of stress.

For most of us, alertness decreases in the early morning, peaks around noon and again in the evening. This is often described as the mid-day dip. Your chronotype also matters:

• Early Morning Type (Lark): Tend to be more alert earlier.
• Evening Type (Owl): Tend to be more alert later.