How important is 8 hours of sleep?

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look up studies on fat loss when participants slept 8 vs 6 and decide for yourself.

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New research by Wang and colleagues (2018) shows us just how detrimental mild sleep deprivation may be for your physique.
For 8 weeks overweight participants slept 1 hour less than normal on working days. They could make up for this in the weekend with 1 hour of extra sleep on both Saturday and Sunday.
Another group of participants could sleep their regular 6 hours and 20 mins every single night.
Both groups were dieting to lose fat, eating around 1400 calories per day.
Seems like the 2 extra hours of sleep during the weekend could not make up for the consistent lack of sleep during the week.
The difference was in the fat-to-muscle loss ratios.
In the sleep deprived, a whopping 83% of total weight loss (~3 kg) was lean mass, while 17% was fat mass.
In contrast, the sound sleepers lost 81% of their weight as fat and 17% as lean mass.
Earlier research supports this stunting of fat loss and muscle preservation: sleeping a little over 5 hours per night more than cut fat loss in half [Nedeltcheva 2010].

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These findings, presented at the 2006 American Thoracic Society International Conference, showed that women who slept 5 hours per night were 32% more likely to experience major weight gain (an increase of 33 pounds or more) and 15% more likely to become obese over the course of the 16-year study, compared to those who slept 7 hours a night.

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The objective of this longitudinal, observational study was to verify whether a favorable change in sleep duration over 6 years could impact objective indicators of adiposity in adults aged 18–64 years. Short-duration sleepers (?6 h per day; n=43) at baseline were divided into two groups: (i) those who increased their sleep duration to a ‘healthy’ length of 7–8 h per day at year 6 (mean increase: 1.52±0.66 h per day; n=23); and (ii) those who maintained their short sleep duration habits (mean change: -0.11±0.38 h per day; n=20). Adult individuals who reported sleeping 7–8 h per day at both baseline and year 6 (n=173) were used as a control group. Change in adiposity indicators for each sleep-duration group was compared by analysis of covariance. We observed that the two short-sleep-duration groups had similar baseline characteristics. However, short-duration sleepers who maintained their short sleep duration experienced a greater increase in body mass index (BMI) (difference: 1.1±0.36 kg m-2, P<0.05) and fat mass (difference: 2.4±0.64 kg, P<0.05) over the 6-year follow-up period than short-duration sleepers who increased their sleep duration, even after adjustment for relevant covariates. We did not observe any significant difference in adiposity changes between the control group and short-duration sleepers who increased their sleep duration. This study suggests for the first time that shifting sleep duration from a short to a healthier length is associated with an attenuation of fat mass gain.

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Insufficient sleep predicts poor weight loss maintenance after 1 year

Results:
After a diet-induced 13.1 kg weight loss, participants with short sleep duration at randomization regained 5.3 kg body weight (p = .0008) and had less reduction in body fat percentage compared with participants with normal sleep duration (p = .007) during the 1-year weight maintenance phase. Participants with poor sleep quality before the weight loss regained 3.5 kg body weight compared with good quality sleepers (p = .010). During the weight maintenance phase, participants undergoing liraglutide treatment displayed increased sleep duration compared with placebo after 26 weeks (5 vs. -15 min/night) but not after 1 year. Participants undergoing exercise treatment preserved the sleep quality improvements attained from the initial weight loss.

Conclusion:
Short sleep duration or poor sleep quality was associated with weight regain after weight loss in adults with obesity.

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Effects of Experimental Sleep Restriction on Energy Intake, Energy Expenditure, and Visceral Obesity


Results.
With sleep restriction vs control, participants consumed more calories (P=0.015), increasing protein (P=0.050) and fat intake (P=0.046). Energy expenditure was unchanged (all P’s >0.16). Participants gained significantly more weight when exposed to experimental sleep restriction than during control sleep (P=0.008). While changes in total body fat did not differ between conditions (P=0.710), total abdominal fat increased only during sleep restriction (P=0.011), with significant increases evident in both subcutaneous and visceral abdominal fat depots (P=0.047 and P=0.042, respectively).

Conclusions.
Sleep restriction combined with ad libitum food promotes excess energy intake without varying energy expenditure. Weight gain and particularly central accumulation of fat indicate that sleep loss predisposes to abdominal visceral obesity.

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Conclusions
Obtaining extra sleep during the weekend to recover from lost sleep during the workweek is a common self-selected sleep strategy employed by adults. We found that healthy adults exhibited increased caloric intake, had greater consumption of saturated fat, and had substantial late-night eating during repeated exposures to sleep restriction regardless of the number of intervening recovery sleep nights. Importantly, our findings indicate humans do not acclimate to sleep restriction with repeated exposures and that one, three or five nights of catch-up sleep is not sufficient for preventing future hyperphagic responses to sleep restriction. These novel findings are timely, given the worldwide obesity epidemic and the large percentage of adults who consistently sacrifice sleep time for other waking behaviors.

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For 8 weeks overweight participants slept 1 hour less than normal on working days. They could make up for this in the weekend with 1 hour of extra sleep on both Saturday and Sunday.
Another group of participants could sleep their regular 6 hours and 20 mins every single night.
Both groups were dieting to lose fat, eating around 1400 calories per day.
Seems like the 2 extra hours of sleep during the weekend could not make up for the consistent lack of sleep during the week.
The difference was in the fat-to-muscle loss ratios.
In the sleep deprived, a whopping 83% of total weight loss (~3 kg) was lean mass, while 17% was fat mass.
In contrast, the sound sleepers lost 81% of their weight as fat and 17% as lean mass.

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Both groups were dieting to lose fat, eating around 1400 calories per day.

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