While 犯困 (fànkùn) might seem like a simple term, its usage touches upon a subtle but important cultural norm in China: the acceptance of the body's natural rhythms. In many Western, particularly American, work cultures, feeling drowsy in the afternoon is often seen as a problem to be “fixed” with caffeine or by “powering through.” It can be perceived as a lack of discipline or engagement. In China, however, feeling 犯困—especially after lunch—is widely accepted as a normal biological response. This leads to the prevalence of the 午睡 (wǔshuì), or the afternoon nap. In many schools, students are expected to nap at their desks for 30-60 minutes after lunch. It's also common in many traditional offices and state-owned enterprises for employees to take a short nap. Therefore, saying “我犯困了” (Wǒ fànkùn le) is not an admission of weakness or unprofessionalism. It's a simple statement of fact, a trigger for a culturally-sanctioned solution: taking a rest to restore one's energy (精神, jīngshen). This aligns with a broader cultural value of balance and working in harmony with one's body, rather than constantly fighting against it.
犯困 is an extremely common, informal term used in daily conversations. You will hear it and use it constantly.