Smaller persons lose heat faster because their surface area is greater than their overall volume. So, if they're outside in the sun, they'll heat up more quickly. Skinny people also tend to burn more calories per unit of time because they have more muscle tissue that needs to be heated up to make sure it's working hard. Fat people don't need to work as hard because they can soak up some of the excess energy from the environment.
The other factor involved with loss of body heat is how well your skin conducts heat. The darker your skin color, the less heat you will be able to conduct away from your body. However, if you are white or pale colored, you will be able to lose much more heat than a black person would under similar conditions.
In conclusion, smaller people will lose heat faster because they have more surface area relative to their volume, and they will also lose heat faster because they have less effective skin color to reduce their temperature. White or pale colored people will still lose heat faster than black people, but by less percentage points than those last two factors would suggest.
However, O'Brien points out that numerous other variables besides subcutaneous fat influence the pace at which we cool. (It's common for women to feel cooler than males; average body size may play a role.) And small bodies can't store as much heat under their skin or within their organs, so they must expend more energy maintaining their temperature.
The answer to your question "do skinny people get colds easily?" is yes, they do. Just like everyone else does. Skinny people and obese people alike are susceptible to coming down with viruses such as the flu. Having less body mass means you burn through fuel faster when you exercise, so functional fitness is important for anyone, not just overweight individuals.
In conclusion, yes, skinny people can get sick too. But only if they lack essential nutrients in their diet, aren't able to keep themselves warm or cool enough during hot days, don't work out often enough, and so on. There are many factors beyond our control that can lead to us getting sick. The same goes for thin individuals - there are many factors other than body weight that can affect how easily they catch illnesses.
Exhaustion Due to Heat This is mostly due to the fact that taller (and heavier) persons create more body heat. If they produce it quicker than they can eliminate it, like during hard activity, this might result in heat stroke or heat exhaustion. These conditions are usually recognized by symptoms such as headache, fatigue, hot flashes, and dizziness.
Tiredness From Work This is probably the most common cause of daytime fatigue for people who do not suffer from sleep disorders. For example, someone who works on a construction site all day long will be very tired after work because they don't get enough rest. People who work in jobs that require them to move around a lot also experience daytime fatigue because they aren't getting adequate recovery time between tasks.
Nighttime Fatigue This type of fatigue occurs when you wake up feeling exhausted even though it was supposed to be your day off. Most likely, you have become used to sleeping eight hours per night but since your body is still processing everything that happened during the day, you need more sleep. Some people who suffer from insomnia may find relief in a caffeine-free diet drink or energy bar before bed.
Drowsiness While Sitting For many people, sitting for too long can lead to feelings of exhaustion. This is especially true if you are used to standing or walking much of the day.