Can you briefly explain how our body is functioning (or functioning less) when we're tired?

Insufficient sleep results naturally in a physiologic drive to sleep that is sometimes beyond the control of the individual. Mental alertness is hindered, resulting in slower response times, poor functionality at work, and risk of injury or accident. Chronic sleep deprivation can additionally impact hormonal levels, causing a rise in cortisol (stress hormone) and a disruption in the balance of leptin & ghrelin (which impact satiety and hunger). Thus, inadequate sleep over the long-term can result in anxiety, depression, irritability, obesity, and metabolic illness.

Why is sleep so important — what does it do/provide to our body?

There are various theories as to the primary role(s) of sleep, and the precise physiology isn’t entirely understood. We know that it is more than just a “sedation” period, and in fact common sedatives like alcohol are well-known to interfere with, rather than enhance, sleep quality. It is believed that during sleep the body is offered a restoration period to repair itself physically and even psychologically. Neurochemical toxins built up during the day are thought to likely be cleared during the sleep state, while new neuron connections are formed within the brain. Memory consolidation, cognitive function, optimized metabolism, emotional health, and normal physiologic organ function are all deemed to be dependent on adequate sleep.

Why do we feel tired in the morning—even if we're well slept?

A sense of next-day rejuvenation is based not only on the duration of sleep, but also on sleep quality (largely related to the amount of uninterrupted time spent in deeper sleep stages). Alcohol, certain medications, and some medical conditions such as sleep apnea can result in poor sleep quality even if sleep duration is perceived to be adequate. In addition, a sense restfulness (or lack thereof) upon awakening is dependent upon the stage of sleep preceding the arousal. If an individual is abruptly awoken during the depths of late-stage, intensive sleep (during deep sleep or REM), they are likely to feel much groggier than if they were to naturally awaken during a lighter stage of sleep. This is why it is typically advised that naps not be longer than about 20-30 minutes (limited to earlier, lighter sleep stages) so as to avoid a hangover effect.

What are 3-5 ways to wake yourself up when you're feeling tired?

Allowing natural light in first thing in the morning is a great way to maintain circadian rhythm patterns that stimulate arousal.

A brief burst of moderate exercise (eg a brisk walk) can frequently shift the body out of its lethargic state. Energy begets energy, they say, and this is most definitely true for humans.

While chronic, habitual overuse of caffeine has multiple adverse consequences, a cup of coffee can indeed counteract some of the impact of a bad night’s sleep. This is not to encourage caffeine dependence, but certainly there is a role for limited use in the context of short-term sleep deprivation (eg to make it through an important meeting after a single night of tossing and turning).