From the moment we are born until late in life, our sleep patterns shift dramatically, much of which can be explained by what scientists call the two-process model. This concept describes how two forces work together to regulate when we fall asleep and wake up: sleep pressure, the urge to sleep that grows the longer we stay awake, and our circadian rhythm, which is our body’s built-in daily clock responding to the cycle of day and night. For infants, these forces operate differently compared to teens and older adults, shaping the rhythm and timing of their sleep in ways that are predictable but unique to each stage of life.
In infancy, sleep pressure builds up very quickly. Babies tire easily, which is why they sleep and nap so frequently throughout both day and night. Their circadian rhythm, the clock that eventually helps sort day from night, is only just beginning to form. This leads to irregular and seemingly unpredictable sleep patterns. As their bodies mature, infants gradually start to consolidate sleep at night and rely less on daytime naps, a process that reflects the gradual alignment of their sleep pressure with a maturing circadian system.
The teenage years bring a major shift. During adolescence, the timing of the internal clock moves later, making teens naturally inclined to fall asleep and wake up at much later hours. At the same time, the buildup of sleep pressure is slower than it is in children, so teenagers don’t feel as sleepy in the early evening and may find it easier to stay awake late into the night. This natural delay, combined with modern habits like evening screen use, often clashes with early school start times, leaving many teens chronically underslept and groggy in the morning.
For older adults, the pattern shifts once again. The circadian clock begins to move earlier, which means older adults tend to get sleepy sooner in the evening and wake up earlier in the morning. The process of sleep pressure building up also becomes slower with age, which can lead to more frequent awakenings during the night and a lighter, more fragmented sleep overall. Additionally, older adults may become less responsive to external cues like light, making it harder for their internal clock to stay aligned with the day-night cycle.
Understanding the two-process model and how these processes evolve with age has important everyday implications. For parents of infants, it explains why strict sleep schedules are often unrealistic and why flexibility helps. In the case of teenagers, it highlights the biological basis for later bedtimes and the value of school schedules that can accommodate those changes. For older adults, this knowledge provides support for strategies like regular morning sunlight exposure and consistent routines to help improve sleep quality.
The two-process model gives scientists, clinicians, and families a shared language, one that helps demystify sleep troubles and tailor solutions to each age group’s needs.
