If you work with a team, chances are your inbox is often flooded with invitations. Internal meetings, client conference calls, the occasional lunch request. Assuming you have some control over your calendar, how you respond to these offers generally depends on two factors: the value of attending the meeting and your availability.

Rarely, however, do most consider a third factor in our decision-making criteria: the time of day when you are at your most productive.

By now, you’ve probably noticed that the person you are midway through the afternoon is not the same person who arrived first thing in the morning. Research shows our cognitive functioning fluctuates throughout the day. If you’re like most people, you’ll find that you can get a lot done between 9:00am and 11:00am. Not so at 2:30pm. Later in the day, it often feels like we’re moving at a fraction of our morning pace.

That’s not an illusion. Recent studies have found that on average, people are considerably worse at absorbing new information, planning ahead and resisting distractions as the day progresses.

The reason this happens is not merely motivational. It’s biological. Our bodies run on a circadian rhythm that affects our hormone production, brain wave activities, and body temperature. Each of these variations tinker with our energy level, impacting our alertness and productivity.

Importantly, we don’t all follow identical patterns. While most people do their best work in the morning (and our preference for mornings tends to increase with age), others are night owls who are more productive later in the day. Research suggests that our fondness for morning or evenings isn’t simply a personal preference—it’s directly tied to the time of day when our physical and cognitive abilities peak. And one new study has even found that morning people are more ethical in the morning – and night owls, more ethical later in the day.

To get the most out of every day, you need to guard the hours when you are at your most productive. Think back to yesterday and the day before. At which points of your day did you feel at your most energetic? (If you’re not sure, tools like RescueTime can help.) Chances are, these are times with the highest productivity potential.

Once you’ve identified high-potential hours, consider treating them differently—for example, by blocking them off on your calendar. This discourages colleagues with access to your availabilities from suggesting these times for meetings. An additional advantage of having high-potential hours blocked off is that it prompts you to think twice before suggesting your own non-essential meetings at that time.

Proactively setting aside your best hours to get work done saves you from having to scramble later on to compensate. Use these hours for working on high-priority projects, making decisions you’ve been avoiding, or initiating a difficult conversation.

And, if you’re the owner of a dull, 10 a.m. staff meeting, do your team a favor and reschedule it for after lunch. The afternoon is when most people’s energy levels naturally dip. Lower energy levels can be disastrous for work that requires deep focus, but is considerably less detrimental in the context of other people. Having others around also naturally increases our alertness levels, helping counteract the slump in energy.

Fatigue, it’s worth noting, is not all bad. In fact, the findings of a 2011 study suggest that when our minds are tired, we are more distractible and less adept at filtering out seemingly irrelevant ideas. The free association that ensues makes “off-peak” hours an ideal time for finding novel solutions.

Ultimately, the best way to schedule is to take our natural energy fluctuations into account. You can maximize your productivity by calibrating activities to the right time of day. If a task requires willpower and complex thinking, plan to do it when you are at your most alert. In contrast, if what you’re after is a fresh perspective, use fatigue to your advantage by looking for solutions when your energy drops.

In either case, protect your best hours. If you don’t do it, who will?