Our bodies are programmed to need sleep. The body indicates sleepiness in two ways: by boosting circulating levels of the neurotransmitter adenosine and by sending signals from the circadian clock, which controls the body’s daily rhythms.

More than 60% of women and men regularly fall short of what an average person’s sleep target should be. It is suggested in latest medical research that for optimum health and function, the average adult should get seven to nine hours of sleep daily. Although each hour of lost slumber goes into the health debit column, we don’t get any monthly reminders that we’ve fallen in the negative. As the sleep debt mounts, the health consequences increase, putting us at growing risk for weight gain, diabetes, heart disease, stroke, and memory loss. In some cases, sleep deficit results from insomnia or other underlying conditions that may require medical attention. But most sleep debt is because of consistently failing to get to bed on time and stay there until we have had enough sleep

Scientifically speaking, as our cells produce power to move us through the day, adenosine is released into the bloodstream and taken up by receptors in the brain region that governs wakefulness (the basal forebrain). There, it acts like a dimmer switch, turning down many of the processes associated with wakefulness, such as attention, memory, and reactions to physical stimuli. As brain levels of adenosine mount, we feel drowsier. This is how caffeine keeps us awake by blocking adenosine receptors in the brain. When we sleep, our energy needs fall, and the level of circulating adenosine drops. After a good night’s sleep, the level is at its lowest, and we are most alert.

The circadian clock regulates all body functions — not just the pattern of sleeping and waking during the 24-hour cycle; but also fluctuations in body temperature, blood pressure, and levels of digestive enzymes and various hormones. Most of us experience a major “sleepiness” peak between 12 a.m. and 6 a.m. and a minor one between 2 p.m. and 4 p.m. Of course, individuals vary. The early birds among us might be ready for bed at 9 or 10 p.m. and be up at 5 a.m. While some night owls don’t fall asleep until well after midnight and prefer sleeping until noon.

Although sleep doesn’t trump food and water in the hierarchy of physical needs, the human body cannot live without it. Given the ethical limits on research involving human subjects, scientists have no direct evidence on how extended sleeplessness — that is, beyond a few days — affects human beings.

In a landmark study of human sleep deprivation, University of Chicago researchers followed a group of student volunteers who slept only four hours a night, for six consecutive days. The volunteers developed higher blood pressure and higher levels of the stress hormone cortisol. The sleep-deprived students also showed signs of insulin resistance — a condition that is the precursor of type 2 diabetes and metabolic slowdown. However, all the changes were reversed when the students made up the hours of sleep they had lost. The Chicago research helps to explain why chronic sleep debt raises the risk of obesity, heart disease, stroke, and diabetes.

Getting the right amount of sleep is just as important for health as diet and exercise are. To that end, we offer the following advice:

Make-up for lost sleep: If you missed 10 hours of sleep over the course of a week, add three to four extra sleep hours on the weekend and an extra hour or two per night the following week until you feel re-energized.

Maintain a routine: Once you have determined how much sleep you really need, incorporate it into your daily schedule and try to go to bed and get up at the same time every day to train your biological sleep clock.