Saunas have long been valued in societies around the world for ability to detoxify the body and promote vitality. Recently, however, research seems to indicate that the benefits of time spent in a sauna might go even deeper.
The ways by which a sauna promotes detoxification are relatively straightforward. Perspiration is a major pathway by which the body excretes toxins, so sweating as much as you do in a sauna is naturally going to improve detoxification.
High temperatures found in a sauna also cause increased blood-flow into tissues via vasodilation, which also aid your body in flushing out and neutralizing toxins that sit stagnant in skeletal muscles and other tissues. This effect is amplified when a sauna session is interrupted by a cold plunge, which has the opposite (vasoconstricting) effect. The result is blood moving dramatically in and out of tissues throughout the body and flushing out toxins in the process.
Recent studies have shown that saunas can also promote another mechanism of detoxification. The unique stress the body experiences in the heat of a sauna has been shown to raise levels of several of the body's endogenous detoxification enzymes, including catalase (CAT) and superoxide dismutase (SOD). 
Human Growth Hormone (hGH): Our Body's Vitality Hormone
First, let’s look at what hGH is and why it's important in the body. hGH is a peptide hormone that sits at the top of a long cascade of hormones related to growth, cell reproduction and cellular regeneration.
hGH is produced and regulated by the pituitary gland but generally does not act directly on other tissues in the body. Rather, hGH triggers the release of secondary growth-related hormones (such as IGF-1 from the liver) that account for the majority of the significant effects seen when hGH levels are elevated.
During childhood and adolescence, hGH plays a central role the growth of our bodies and brains. Later in life, the role of hGH becomes one of maintaining youthfulness and vitality.
hGH is also famously the hormone responsible for catalyzing muscle growth in adults, which is why it has long been popular with athletes and body-builders - and has been on the banned substances list of the regulatory bodies for most sports since the early 2000's.
Largely owing to the regenerative processes triggered by hGH, nearly everyone stands to benefit from maintaining healthy circulating levels of hGH. If circulating levels can be raised naturally, if even just temporarily, this would also confer benefits and be of interest.
Sauna Treatment And hGH
It's widely known among biochemists and exercise physiologists that certain types of exercise can raise circulating human growth hormone levels. What's less widely known is that sitting in the sauna can have a comparable effect under certain conditions.
The initial study that revealed the sauna-hGH correlation is actually quite old. A Finnish team published a paper in 1976 that looked at hGH levels in 55 healthy individuals before and after a sauna session. It found that hGH levels were on average 140% higher immediately following a sauna session than they were before the session. hGH levels returned to normal levels after an hour for most participants, but the temporary spike was large enough to make the findings significant. 
Scattered studies over the following 30 years used slightly different experiment protocols and participant demographics, and all found similar elevations in circulating hGH levels following a sauna treatment.
Maximizing The hGH Response
The unique experiment protocols from the different studies over the years have given us some insights as to how to maximize the magnitude of the hGH release from a sauna session.
First, it's important to note that hGH levels tend to drop off significantly for most people around the age of 50. Given this, it doesn't come as much of a surprise that a study in the mid 1980’s found little to no elevation of circulating hGH in men ages 49-66 when given the same sauna treatment that produced significant elevations in hGH in men ages 31-46. 
The trick to maximizing hGH response seems to be subjecting the body to exactly the right amount of heat-induced stress. The temperature, duration and frequency of the sauna session all influence this calculation.
Several studies have shown that the body will adapt to the stress of the sauna when it's used every day. For this reason, the effect on hGH levels diminishes significantly after three consecutive days of sauna treatment.  The studies that showed the greatest continued hGH response followed a format of sauna treatments every second day.
The duration of a sauna treatment had significant bearing on the magnitude of the hGH response, as did the presence of a break in the session. Most studies showed that session durations of ~30 minutes produced the greatest elevation in circulating hGH levels.
A 2007 study used both 30 minute continuous sessions and 45 minute sessions 5-minute break at the midpoint. The participants in the first group (30 continuous minutes) showed much greater elevations in hGH levels. 
Temperatures used in most studies were around 80 degrees Celsius (176F) with humidity generally around 5-20%. No study as yet has tracked the relationship between session duration and temperature and magnitude of hGH elevation. However, as higher temperatures will create greater stress in the body, it's reasonable to expect that the duration required to produce the same effect would go down as temperature increases.
I'm sure it will have occurred to some of you - sauna-induced elevation of hGH could be quite beneficial for athletes and others who train intensely (who's bodies are actively rebuilding muscle and recovering).
There is a period of around 30-60 minutes immediately following a workout when your body will be doing the most active rebuilding of muscle and replenishing of glycogen (the muscles' energy storage). This is why it's so critical to give your body the exact nutrition it needs as soon as possible after a workout.
Raising circulating hGH during this window can potentially further bolster the already-heightened rebuilding and repairing your body is doing in this window. This support of your body's rebuilding efforts will translate to greater gains in muscle and fitness, as well as being less sore the following days.
If you're lucky enough to have access to a sauna after your workouts, taking 30 minutes or so to support your body's rebuilding efforts is probably a good idea.
Of course, the benefits of raising circulating hGH levels are hardly limited to athletes. As I mentioned above, hGH has a regenerative effect on nearly every tissue, muscle and organ in the body. For this reason, regular sauna sessions is a tremendously powerful practice for maintaining a youthful body and appearance.