Why some employees struggle to work in morning shifts while others dislike later hour shifts, Harvard scholar explains the biological reason

Not everyone starts the workday with the same level of energy. While some employees are most productive in the early hours of the morning, others find it difficult to focus until later in the day. These differences are often misunderstood as a matter of discipline, motivation, or work ethic. However, according to a Harvard-affiliated psychologist specializing in behavioral sleep medicine, biology may be a much bigger factor than many people realize.

According to Harvard.edu, Michael A. Grandner, a clinical psychologist who studies sleep and circadian rhythms, says that people are guided by internal biological clocks that influence when they feel alert, focused, and ready to perform demanding tasks. These natural patterns can help explain why some workers struggle with morning shifts while others find late-evening schedules exhausting.

Understanding the body's internal clock

At the center of the discussion is the circadian rhythm, the body's natural 24-hour cycle that regulates physical, mental, emotional, and behavioral changes. This internal clock is influenced by environmental cues such as light, darkness, and temperature, causing energy and attention levels to fluctuate throughout the day.


Not everyone follows the same schedule, though. According to Grandner, these differences are known as chronotypes. Around 15% of people are "morning larks," who naturally wake up early and perform best in the first part of the day. Another 15% are "night owls," whose energy peaks later in the day. The remaining 70% fall somewhere in between.

Importantly, Grandner notes that these patterns are not simply personal preferences. "Chronotype is genetically determined; it is not a preference."

That means an employee who struggles with an early shift may not be lazy or unmotivated. Their biological clock may simply be operating on a different timetable.

Why timing affects performance

Research has shown that people perform best when their work aligns with their chronotype. Scientists call this the "synchrony effect."

According to Grandner, the synchrony effect influences cognitive abilities such as attention, concentration, problem-solving, decision-making, and self-control. When people work during their natural peak hours, tasks often require less effort and mistakes are less likely.

The opposite can also be true. Employees who regularly work outside their optimal hours may find themselves feeling more tired, less focused, and more frustrated even when they are putting in the same amount of effort.

Grandner shared his own experience as an early morning chronotype. Reflecting on work that extends into the evening, he wrote, "I accept the reality that I am working against my early morning chronotype; that I will expend greater effort for decreased performance and increased frustration, mistakes, and lackluster interpersonal interactions."

He also recalled teaching evening classes early in his career, describing them as "the absolute worst time for an early morning type."

What workers can do

While many employees cannot choose their schedules, Grandner recommends paying attention to when important tasks feel easiest to complete. Understanding personal energy patterns can help workers schedule demanding activities during their most productive hours whenever possible.

For those who must work outside their preferred times, he suggests strategies such as getting exposure to natural light, staying physically active, eating balanced meals, and maintaining proper hydration.

His broader message is that differences in workplace performance are not always about effort. In many cases, employees who dislike morning shifts or late-hour schedules may simply be responding to biological rhythms that science has shown are deeply rooted in human behavior.