Purpose To examine the effects of 24-h manipulations in energy availability (EA), through altering energy intake (EI) or exercise energy expenditure (EEE), on sleep characteristics and nocturnal heart rate variability (HRV). Methods Twenty endurance athletes (10 females, 10 males) undertook five 24-h periods of EA manipulation, separated by 8 d, in a randomized Latin square design. Following 24 h of standardized high EA (45 kcal·kg-1 FFM·d-1), athletes then followed a diet providing low EA (15 kcal·kg-1 FFM·d-1) either with/without exercise (LEAEX/REST), high EA (45 kcal·kg-1 FFM·d-1) with/without exercise (HEAEX/REST), and excess EA (75 kcal·kg-1 FFM·d-1) without exercise (GEA). Sleep and overnight HRV were assessed using Somfit and Oura ring devices, respectively, pre- and post-EA manipulation. Results Total sleep duration was extended by 53-61 min following LEAEX versus non-exercise conditions: GEA (P = 0.002), HEAREST (P = 0.003), and LEAREST (P = 0.020). Time in bed was also increased during LEAEX compared with GEA (P = 0.006) and HEAREST (P = 0.023), but without alterations to sleep efficiency between conditions (P = 0.613). Sleep onset latency increased by 8 min following LEAREST compared with GEA (P = 0.012), whereas there were no other alterations to objective/subjective sleep characteristics between conditions (all P > 0.05). There was no difference in overnight HRV following the 24-h EA manipulation (P > 0.05). However, mean overnight HR was lower following LEAREST and HEAREST (-4 ± 3 and -2 ± 3 bpm), compared with LEAEX (+1 ± 3 bpm), HEAEX (0 ± 3 bpm), and GEA (0 ± 3 bpm, all P < 0.01). There was no difference between sexes in the response to EA manipulation across all variables. Conclusions Twenty-four hours of exercise-induced LEA extended total sleep time, whereas diet-induced LEA reduced mean overnight HR. The responses to EA manipulations did not differ between sexes.
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