Associations of excessive smartphone duration and unlock frequency with non-suicidal self-injury in college students

Table 1 Characteristics of participants, n (%)

Characteristics	Categories	Participants	NSSI		P value
			No	Yes
Gender	Male	6170 (34.6)	6029 (34.7)	141 (30.7)	0.08
	Female	11681 (65.4)	11362 (65.3)	319 (69.3)
Grade	1st	6155 (34.5)	5979 (34.4)	176 (38.3)	< 0.001
	2nd	4555 (25.5)	4415 (25.4)	140 (30.4)
	3rd	4688 (26.3)	4581 (26.3)	107 (23.3)
	4th+	2453 (13.7)	2416 (13.9)	37 (8.0)
Race	Han	17126 (95.9)	16691 (96.0)	435 (94.6)	0.16
	Others	725 (4.1)	700 (4.0)	25 (5.4)
Registered permanent residence	Rural	7674 (43.0)	7511 (43.2)	163 (35.4)	0.001
	Urban	10177 (57.0)	9880 (56.8)	297 (64.6)
Siblings	No	6154 (34.5)	5983 (34.4)	171 (37.2)	0.24
	Yes	11697 (65.5)	11408 (65.6)	289 (62.8)
Maternal educational attainment	Middle school or under	9981 (55.9)	9745 (56.0)	236 (51.3)	0.06
	High school	4163 (23.3)	4053 (23.3)	110 (23.9)
	College or above	3707 (20.8)	3593 (20.7)	114 (24.8)
Parental educational attainment	Middle school or under	8656 (48.5)	8456 (48.6)	200 (43.5)	0.01
	High school	4438 (24.9)	4330 (24.9)	108 (23.5)
	College or above	4757 (26.6)	4605 (26.5)	152 (33.0)
Family disaster	Yes	1620 (9.1)	1539 (8.8)	81 (17.6)	< 0.001
	No	16231 (90.9)	15852 (91.2)	379 (82.4)
Hospitalization experience	Yes	1700 (9.5)	1600 (9.2)	100 (21.7)	< 0.001
	No	16151 (90.5)	15791 (90.8)	360 (78.3)
Failed exams	Yes	7360 (41.2)	7066 (40.6)	294 (63.9)	< 0.001
	No	10491 (58.8)	10325 (59.4)	166 (36.1)
Failed relationships	Yes	3401 (19.1)	3207 (18.4)	194 (42.2)	< 0.001
	No	14450 (80.9)	14184 (81.6)	266 (57.8)
Smoking	Yes	3165 (17.7)	3019 (17.4)	146 (31.7)	< 0.001
	No	14686 (82.3)	14372 (82.6)	314 (68.3)
Drinking	Yes	4401 (24.7)	4185 (24.1)	216 (47.0)	< 0.001
	No	13450 (75.3)	13206 (75.9)	244 (53.0)
Rational diet	Yes	2397 (13.4)	2349 (13.5)	48 (10.4)	0.07
	No	15454 (86.6)	15042 (86.5)	412 (89.6)
Physical exercise	Moderate or high	3694 (20.7)	3596 (20.7)	98 (21.3)	0.79
	Low	14157 (79.3)	13795 (79.3)	362 (78.7)
Smartphone duration (hours/week), mean ± SD		49.1 ± 29.9	48.9 ± 29.8	56.9 ± 31.5	< 0.001
Smartphone duration (hours/week)	0-21	4149 (23.2)	4069 (23.4)	80 (17.4)	< 0.001
	21-42	2922 (16.4)	2865 (16.5)	57 (12.4)
	42-63	4626 (25.9)	4508 (25.9)	118 (25.7)
	≥ 63	6154 (34.5)	5949 (34.2)	205 (44.6)
Smartphone unlock frequency (times/week), mean ± SD		390.9 ± 278.2	389.5 ± 276.4	437.1 ± 327.4	< 0.001
Smartphone unlock frequency (times/week)	0-50	4062 (22.8)	3986 (22.9)	76 (16.5)	< 0.001
	50-150	5972 (33.5)	5840 (33.6)	132 (28.7)
	150-400	4249 (23.8)	4128 (23.7)	121 (26.3)
	≥ 400	3568 (20.0)	3437 (19.8)	131 (28.5)

NSSI: Non-suicidal self-injury.

Table 2 Association between smartphone use and non-suicidal self-injury

Smartphone use	Categories	OR (95%CI)
		Model 11	Model 22	Model 33
Smartphone duration, continuous variable	Per 21 hours/week	1.17 (1.10, 1.25)	1.15 (1.08, 1.23)	1.16 (1.08, 1.24)
Smartphone duration, categorical variable	0-21 hours/week	1 (reference)	1 (reference)	1 (reference)
	21-42 hours/week	1.08 (0.76, 1.53)	1.17 (0.82, 1.65)	1.09 (0.77, 1.54)
	42-63 hours/week	1.31 (0.98, 1.76)	1.36 (1.02, 1.83)	1.22 (0.91, 1.65)
	≥ 63 hours/week	1.63 (1.25, 2.13)	1.60 (1.23, 2.11)	1.60 (1.23, 2.11)
	Ptrend value	< 0.001	< 0.001	< 0.001
Smartphone unlock frequency, continuous variable	Per 50 times/week	1.04 (1.02, 1.05)	1.031 (1.02, 1.05)	1.03 (1.01, 1.04)
Smartphone unlock frequency, categorical variable	0-50 times/week	1 (reference)	1 (reference)	1 (reference)
	50-150 times/week	1.12 (0.85, 1.50)	1.07 (0.80, 1.43)	0.98 (0.73, 1.31)
	150-400 times/week	1.45 (1.09, 1.96)	1.41 (1.05, 1.90)	1.33 (0.99, 1.79)
	≥ 400 times/week	1.77 (1.32, 2.37)	1.63 (1.21, 2.19)	1.48 (1.11, 2.00)
	Ptrend value	< 0.001	< 0.001	0.001

¹Model 1, adjusted for gender, grade, race, registered permanent residence, siblings, and parental educational attainment.

²Model 2, adjusted for gender, grade, race, registered permanent residence, siblings, parental educational attainment, current smoking, current drinking, physical activity, and rational diet.

³Model 3, adjusted for gender, grade, race, registered permanent residence, siblings, parental educational attainment, current smoking, current drinking, physical activity, rational diet, family disasters, hospitalization experience, failed exams, and failed relationships.

OR: Odds ratio; CI: Confidence interval.

Table 3 Gender-specific association between smartphone use and non-suicidal self-injury

Smartphone use	Categories	OR (95%CI)1		P value for interaction
		Male	Female
Smartphone duration, continuous variable	Per 21 hours/week	1.19 (1.06, 1.33)	1.14 (1.05, 1.23)	0.67
Smartphone duration, categorical variable	0-21 hours/week	1 (reference)	1 (reference)	0.03
	21-42 hours/week	0.90 (0.50, 1.59)	1.40 (0.89, 2.17)
	42-63 hours/week	0.82 (0.47, 1.40)	1.72 (1.20, 2.50)
	≥ 63 hours/week	1.71 (1.11, 2.68)	1.64 (1.17, 2.34)
	Ptrend value	0.011	0.007
Number of smartphones unlocks, continuous variable	Per 50 times/week	1.02 (0.10, 1.04)	1.04 (1.02, 1.05)	0.20
Number of smartphones unlocks, categorical variable	0-50 times/week	1 (reference)	1 (reference)	0.17
	50-150 times/week	0.71 (0.43, 1.19)	1.29 (0.90,1.85)
	150-400 times/week	1.08 (0.65, 1.81)	1.60 (1.11, 2.32)
	≥ 400 times/week	1.08 (0.65, 1.82)	1.99 (1.39, 2.88)
	Ptrend value	0.232	< 0.001

¹These analyses were adjusted for grade, race, registered permanent residence, siblings, parental educational attainment, current smoking, current drinking, physical activity, rational diet, family disasters, hospitalization experience, failed exams, and failed relationships.

OR: Odds ratio; CI: Confidence interval.