The Science of Rest

Understanding Energy and the Science of Rest

Written by: Sydney Hillis, MScOT Student UofT

Maybe you sleep eight hours and wake up exhausted or you spend Sunday resting but still feel depleted on Monday. You may finally have an opportunity to relax, but your brain seems incapable of switching off.

When this happens, the common advice is to get more rest. But fatigue is more complicated than how many hours we sleep and recovery requires more than simply stopping activity. 

Fatigue versus Sleepiness

To understand your energy and the benefits of rest, it is important to understand the distinction between sleepiness and fatigue. One important distinction is between sleepiness and fatigue.

Sleepiness is the biological drive to sleep whereas fatigue is broader and can include physical heaviness, brain fog, reduced concentration, emotional exhaustion and a decreased capacity to manage everyday demands.

This is why someone can be fatigued without feeling sleepy and why sleeping longer does not necessarily resolve fatigue.

Energy is influenced by much more than physical activity. Pain, hormonal changes, emotional stress, decision-making, sensory stimulation, caregiving and cognitive demands all require resources from the brain and body. Because of this, a person can spend much of the day resting and still be completely exhausted.

Two Systems that Control Sleep

Sleep is partly regulated by two interacting biological processes.

The first is sleep drive and sleep pressure. A chemical called adenosine gradually accumulates in the brain while we are awake which increases our drive to sleep. Sleep then reduces this pressure.

The second is our circadian rhythm (our approximately 24-hour internal clock). Light exposure is one of its strongest signals and helps regulate hormones involved in alertness and sleep, including melatonin. This explains why simply going to bed earlier does not always result in earlier sleep. You may be physically tired, but if your circadian system is still promoting wakefulness, falling asleep can be difficult.

It also helps explain why consistent wake times, daytime light exposure and regular activity can influence sleep, sometimes as much as focusing on bedtime itself.

Rest Isn't Always Restorative

Rest works best when it matches the type of demand your system has been managing.

If you have spent hours concentrating, making decisions and switching between tasks, you may need cognitive rest. If you have been managing noise, screens and constant interruptions, reducing sensory input may be more restorative.

If your body has been under prolonged stress, physically stopping activities (what we imagine rest to be) may not immediately switch off your stress response. The autonomic nervous system helps regulate processes such as heart rate, breathing and digestion. When we are repeatedly responding to stressors, the body may remain in a more activated state even when the immediate demand has stopped. This can create the frustrating experience of being exhausted but unable to relax. It is also why scrolling on the couch may feel like rest physically while not fully restoring energy as our phones continue to provide significant cognitive and sensory stimulation.

Recovery

Interestingly, the most restorative activity is not always the least active one.

Low- to moderate-intensity movement can support sleep, mood and regulation. Enjoyable activities can provide psychological recovery. Social connection can be restorative for one person, while quiet time alone may be what another person needs after spending all day responding to others. From an occupational therapy perspective, this is an important distinction: activities can consume energy, restore energy, or sometimes do both. What activities are which category may change person to person and day by day.

Rather than simply asking “How much am I doing?” it can be more useful to ask “What types of demands am I experiencing, and what helps me recover from them?”

Sleep and Phases of Life

Sleep is not static across the lifespan, and hormonal changes can significantly influence it.

During pregnancy, progesterone, physical discomfort, reflux, increased urination and changes in breathing can alter sleep quality and increase nighttime waking.

During postpartum, sleep often becomes fragmented. This matters because sleep occurs in repeating stages and cycles throughout the night. Frequent awakenings can interrupt these cycles, meaning that eight total hours accumulated in smaller pieces may not feel equivalent to eight hours of relatively uninterrupted sleep.

During perimenopause and menopause, changes in estrogen and progesterone interact with systems involved in temperature regulation, mood and sleep. Hot flashes and night sweats can directly cause awakenings, while insomnia and sleep-disordered breathing can also become more common during this stage of life.

Sleep architecture also changes with age. Deep sleep generally decreases, nighttime awakenings become more common and the timing of the body's internal clock can shift. So when sleep suddenly feels different, it is not necessarily because someone has developed “bad sleep habits.” Biology and life stage matter too.

Rest before Empty

Many people use rest reactively: we keep going until we are exhausted and then stop.

For people experiencing pain or fatigue, this can contribute to a boom-and-bust cycle. Doing significantly more when energy is available, experiencing a flare or crash, needing prolonged recovery and then repeating the cycle once they feel better.

Recovery can sometimes be more effective when it is incorporated before complete exhaustion. This might mean alternating cognitively and physically demanding tasks, taking shorter breaks throughout the day, or noticing early signs that your capacity is changing rather than waiting until your body forces you to stop. The goal is not to avoid activity but to make activity more sustainable.

Understanding your Energy

When fatigue begins interfering with daily life, asking someone to simply “rest more” misses a large part of the picture. A more useful starting point may be: What is using your energy? What is restoring it? And are the two currently in balance?

Occupational therapists can help identify patterns between sleep, symptoms, routines, responsibilities and energy throughout the day. This can include looking at pacing, activity patterns, environmental demands, meaningful activities and opportunities for recovery. Effective rest is not necessarily about doing nothing but it is about giving your brain and body the type of recovery they actually need.